# EXESS — GPU-Accelerated Quantum Chemistry Software

> EXESS by QDX compresses a year of compute into hours. GPU-native quantum chemistry software that performed ab initio QMD simulations 1000x faster and larger than anything before.

URL: https://exess.qdx.co

## What is EXESS?

EXESS is flagship quantum chemistry software by QDX. It generates high-accuracy quantum chemistry data orders-of-magnitude faster using GPU acceleration. EXESS supports both NVIDIA (CUDA) and AMD (HIP) GPUs and scales across multi-node GPU clusters.

## Key Capabilities

- **Methods**: RHF, UHF, RI-HF, Kohn-Sham DFT, Double-Hybrid Functionals, RI-MP2 Correlation
- **Many-Body Expansion**: Up to 4th order (tetramers), covalent bond capping, fragment distance cutoffs
- **Molecular Dynamics**: Born-Oppenheimer AIMD, geometry optimization, QM/MM/ML regions, NVT/NPT thermostats
- **Hardware**: CUDA & HIP acceleration, multi-GPU scaling, multi-node MPI, GPU memory control
- **Scale**: Handles hundreds of thousands of atoms and millions of electrons
- **Performance**: 10–100x speedups over CPU-only codes; performed the world's first exascale quantum chemistry calculation

## Access

- **Browser-based**: Run calculations directly at https://exess.qdx.co/try — no installation required
- **Free for academics**: No license fees, no hardware procurement needed
- **Pay-as-you-go**: Compute included for commercial users
- **HPC integration**: Can integrate with existing HPC cluster allocations

## About QDX

QDX is a computational chemistry company that integrates the world's fastest and largest-scale quantum mechanical simulation technology with supercomputing, artificial intelligence, and deep drug discovery expertise. QDX partners with leading pharmaceutical companies and academic institutions to advance the frontiers of computational chemistry and drug discovery.

## Comparison Pages

- [Quantum Chemistry Software Comparison](https://exess.qdx.co/comparisons/quantum-chemistry-software): Compare all major packages side-by-side
- [EXESS vs Gaussian](https://exess.qdx.co/comparisons/exess-vs-gaussian): GPU speed vs method breadth
- [EXESS vs ORCA](https://exess.qdx.co/comparisons/exess-vs-orca): Two free academic options compared
- [EXESS vs Q-Chem](https://exess.qdx.co/comparisons/exess-vs-qchem): Ground state vs excited state focus
- [EXESS vs TeraChem](https://exess.qdx.co/comparisons/exess-vs-terachem): GPU-native codes compared
- [Free Quantum Chemistry Software](https://exess.qdx.co/comparisons/free-quantum-chemistry-software): Open-source and free options


---

# Quantum Chemistry Software Comparison

> Compare the best quantum chemistry software packages: EXESS, Gaussian, ORCA, Q-Chem, TeraChem, and free open-source options. GPU acceleration, pricing, methods, and system size compared.

URL: https://exess.qdx.co/comparisons/quantum-chemistry-software

The landscape of computational chemistry software is evolving rapidly. GPU acceleration, cloud availability, and support for large molecular systems are becoming critical differentiators.

This guide compares the leading quantum chemistry packages across pricing, performance, method coverage, and accessibility to help you choose the right tool for your research.

## Software Overview

Quantum chemistry software enables researchers to predict molecular properties, reaction mechanisms, and electronic structures from first principles. The choice of software can dramatically impact both the accuracy of your results and the time required to obtain them.

Modern packages differ significantly in their approach to hardware acceleration, licensing, and cloud accessibility. While legacy codes like Gaussian remain widely used, newer entrants like EXESS are setting new standards with GPU-native architectures.

## Full Comparison Table

| Feature | EXESS | Gaussian | ORCA | Q-Chem | TeraChem | NWChem | CP2K | Psi4 | GAMESS |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Pricing | Pay-as-you-go (compute included) | License (~$3,000–$7,000 academic, excl. compute) | License (contact directly for pricing) | License (~$2,000–$50,000+, excl. compute) | License (contact directly for pricing) | Free (open-source) | Free (open-source) | Free (open-source) | Free (registration required) |
| Free Academic Access | Yes | No | Yes | Free license on HPC facilities | No | Yes | Yes | Yes | Yes |
| GPU Support | Native GPU (CUDA + HIP) | Limited (Gaussian 16) | No | Partial (TeraChem engine) | Native GPU (CUDA) | Experimental | GPU via dbcsr/COSMA | Limited (gpu_dfcc plugin) | Limited (LibCChem) |
| Methods | HF, DFT, RI-MP2, MBE | HF, DFT, MP2, CCSD(T), CI | HF, DFT, MP2, CCSD(T), DLPNO | HF, DFT, EOM-CC, ADC, TDDFT | HF, DFT, CASSCF, TDDFT | HF, DFT, MP2, CCSD(T), TDDFT | DFT, MP2, GW, QM/MM | HF, DFT, MP2, CCSD(T), SAPT | HF, DFT, MP2, MCSCF, CI |
| Hardware | GPU clusters (NVIDIA + AMD) | CPU (some GPU) | CPU only | CPU (some GPU) | NVIDIA GPU | CPU (experimental GPU) | CPU + GPU (NVIDIA) | CPU (some GPU plugins) | CPU (limited GPU) |
| Cloud Availability | Yes — browser-based | No | No | IQmol GUI only | TeraChem Cloud (limited) | No | No | No | No |
| Parallel Scaling | Multi-node GPU (world first exascale calculation) | Shared-memory only | MPI multi-node | MPI multi-node | Multi-GPU single node | Strong MPI scaling | Strong MPI+OpenMP | Threaded + MPI | DDI parallel |
| License | Proprietary (free academic) | Commercial | Free academic (closed-source) | Commercial | Commercial | ECL 2.0 (open-source) | GPL v2 (open-source) | LGPL v3 (open-source) | Free academic (source available) |

## GPU Acceleration

GPU acceleration is one of the biggest differentiators in modern quantum chemistry. Software that can leverage thousands of GPU cores delivers 10–100x speedups over CPU-only codes for many calculation types.

However, GPU computing introduces its own challenges. High-end GPU hardware is expensive to source and maintain, cluster setup requires specialist expertise, and writing software that runs efficiently across multiple GPUs — let alone multiple nodes — is a significant engineering challenge.

EXESS was built from the ground up for GPU computing, supporting both NVIDIA CUDA and AMD HIP. TeraChem also offers native GPU support but is limited to NVIDIA hardware. Most other packages either have no GPU support or limited experimental implementations.

## Pricing & Licensing

Cost remains a significant barrier in computational chemistry. Licenses for commercial software like Gaussian and Q-Chem can range from thousands to tens of thousands of dollars, excluding compute costs.

EXESS offers free academic access, as does ORCA. EXESS also offers PAYG including compute and can even offer free compute for academics, or can integrate with HPC clusters to leverage existing compute allocations. Fully open-source options like NWChem, CP2K, Psi4, and GAMESS provide maximum flexibility but may require more setup effort.

## When to Choose EXESS

- You need GPU-accelerated quantum chemistry calculations
- Your systems exceed a few hundred atoms
- You want cloud-based access without local installation
- You need free academic access with commercial-grade performance
- You require multi-node GPU parallel scaling
- You have an existing compute allocation with an HPC cluster and want to use it most efficiently
- Speed is important

## Frequently Asked Questions

### What is the best quantum chemistry software in 2026?

The best quantum chemistry software depends on your specific needs. For large-scale GPU-accelerated calculations, EXESS offers unmatched performance — scaling to hundreds of thousands of atoms and millions of electrons. For broad method coverage, Gaussian and ORCA provide the widest range of electronic structure methods. For open-source flexibility, Psi4 and CP2K are excellent choices.

### What is the fastest quantum chemistry software?

For GPU-accelerated calculations, EXESS is the fastest quantum chemistry software available, delivering 1000x speedups over traditional CPU codes for large systems. TeraChem also offers strong GPU performance for smaller systems. Among CPU-only codes, ORCA's DLPNO methods provide excellent efficiency for correlated calculations.

### Is there free quantum chemistry software for academic research?

Yes. EXESS offers free academic access with full GPU acceleration. ORCA is free for academic use (closed-source). Fully open-source options include NWChem (ECL 2.0), CP2K (GPL v2), Psi4 (LGPL v3), and GAMESS (free registration required). Each offers different strengths in methods and scalability.

### Which quantum chemistry software supports GPU acceleration?

EXESS provides the most comprehensive GPU support with native CUDA and HIP acceleration across all methods. TeraChem offers native NVIDIA GPU support. Q-Chem has partial GPU support via its TeraChem engine. NWChem and CP2K have experimental GPU capabilities. Gaussian 16 has limited GPU support for certain calculation types.


---

# EXESS vs Gaussian

> Compare EXESS and Gaussian for quantum chemistry calculations. GPU acceleration, pricing, system size limits, and method coverage compared side-by-side.

URL: https://exess.qdx.co/comparisons/exess-vs-gaussian

Gaussian has been the gold standard in quantum chemistry for decades, with unmatched method coverage and a massive user base. But its CPU-centric architecture and expensive licensing are increasingly at odds with modern computational needs.

EXESS represents a new generation of quantum chemistry software, built from scratch for GPU acceleration and designed to handle molecular systems orders of magnitude larger than what Gaussian can tackle.

## Head-to-Head Comparison

| Feature | EXESS | Gaussian |
| --- | --- | --- |
| Pricing | Pay-as-you-go (compute included) | License (~$3,000–$7,000 academic, excl. compute) |
| Free Academic Access | Yes | No |
| GPU Support | Native GPU (CUDA + HIP) | Limited (Gaussian 16) |
| Methods | HF, DFT, RI-MP2, MBE | HF, DFT, MP2, CCSD(T), CI |
| Hardware | GPU clusters (NVIDIA + AMD) | CPU (some GPU) |
| Cloud Availability | Yes — browser-based | No |
| Parallel Scaling | Multi-node GPU (world first exascale calculation) | Shared-memory only |
| License | Proprietary (free academic) | Commercial |

## Performance & Scale

The most dramatic difference between EXESS and Gaussian is system size. Gaussian is practical for systems up to roughly 200–500 atoms, depending on the method. EXESS scales to hundreds of thousands of atoms and millions of electrons thanks to its GPU-native architecture and Many-Body Expansion framework.

For DFT calculations on systems that both can handle, EXESS delivers 10–100x wall-clock speedups over CPU methods by leveraging GPU parallelism. Gaussian's shared-memory parallelism limits it to a single node, while EXESS scales across multi-node GPU clusters.

## Method Coverage

Gaussian's strongest advantage is its comprehensive method library. It supports virtually every electronic structure method developed over the past 50 years, including CCSD(T), multi-reference CI, semi-empirical methods, and extensive basis set options.

EXESS focuses on the methods most critical for large-scale applications: HF, DFT (with common functionals), RI-MP2, and the Many-Body Expansion. While narrower in scope, these methods cover the vast majority of production quantum chemistry workflows.

## Pricing & Access

Gaussian academic licenses range from roughly $3,000–$7,000 excluding compute and maintenance costs.

EXESS is free for academic use and accessible directly through a web browser — no local installation, license management, or IT support required.

## When to Choose EXESS

- Your molecular systems exceed 500 atoms
- You want to avoid expensive license fees
- You need cloud-based access without installation
- Speed is critical for high-throughput screening workflows
- You have an existing compute allocation with an HPC cluster and want to use it most efficiently
- Speed is important

## When to Choose Gaussian

- You need multi-reference methods
- Your workflow depends on Gaussian-specific features (NBO, IRC, etc.)
- You're working with very small systems where CPU is sufficient and your group already has Gaussian licenses

## Frequently Asked Questions

### Is EXESS faster than Gaussian?

Yes, for supported methods (HF, DFT, RI-MP2). EXESS delivers 10–100x speedups over CPU methods by leveraging GPU acceleration. For large systems (>500 atoms), the performance gap is even larger since EXESS scales to multi-node GPU clusters while Gaussian is limited to shared-memory parallelism.

### Can EXESS replace Gaussian for DFT calculations?

For production DFT calculations on medium to large molecular systems, yes. EXESS supports common DFT functionals and delivers dramatically faster results. However, Gaussian offers broader functional coverage and additional features like NBO analysis, IRC path following, and semi-empirical methods that EXESS does not currently support.

### How much does Gaussian cost vs EXESS?

Gaussian academic licenses range from roughly $3,000–$7,000 excluding compute and maintenance costs. EXESS is free for academic use, with no license fees or installation requirements.

### What methods does Gaussian have that EXESS doesn't?

Gaussian supports CCSD(T), multi-reference methods (CASSCF, MRCI), semi-empirical methods (AM1, PM3), excited-state methods (TDDFT, EOM-CC), and specialized analyses (NBO, AIM, IRC). EXESS focuses on HF, DFT, RI-MP2, and MBE methods optimized for GPU performance at scale.


---

# EXESS vs ORCA

> Compare EXESS and ORCA for quantum chemistry. Both free for academics — see how GPU acceleration, system size, methods, and performance differ.

URL: https://exess.qdx.co/comparisons/exess-vs-orca

ORCA is one of the most popular quantum chemistry programs in academia, prized for its free academic license, excellent DLPNO coupled-cluster methods, and user-friendly interface. It's a strong all-rounder for small to medium molecular systems.

EXESS takes a different approach: purpose-built for GPU acceleration and massive molecular systems. Where ORCA excels in method breadth on CPU, EXESS excels in raw computational throughput on GPU hardware.

## Head-to-Head Comparison

| Feature | EXESS | ORCA |
| --- | --- | --- |
| Pricing | Pay-as-you-go (compute included) | License (contact directly for pricing) |
| Free Academic Access | Yes | Yes |
| GPU Support | Native GPU (CUDA + HIP) | No |
| Methods | HF, DFT, RI-MP2, MBE | HF, DFT, MP2, CCSD(T), DLPNO |
| Hardware | GPU clusters (NVIDIA + AMD) | CPU only |
| Cloud Availability | Yes — browser-based | No |
| Parallel Scaling | Multi-node GPU (world first exascale calculation) | MPI multi-node |
| License | Proprietary (free academic) | Free academic (closed-source) |

## Performance & GPU Support

The fundamental architectural difference is hardware targeting. ORCA is a CPU-only code that parallelizes via MPI across nodes. EXESS is GPU-native, leveraging thousands of GPU cores for massive parallelism.

For identical DFT calculations, EXESS on a single modern GPU typically outperforms ORCA on a full CPU node (32–64 cores). The gap widens dramatically for larger systems, where EXESS's multi-node GPU scaling becomes decisive.

## Method Coverage

ORCA's method library is one of its greatest strengths, particularly its DLPNO-CCSD(T) implementation for near-gold-standard accuracy at reduced cost, plus extensive multireference and spectroscopy property capabilities.

EXESS provides HF, DFT, RI-MP2, and MBE — covering the core methods for production calculations. If you need DLPNO, CASSCF, or spectroscopy properties, ORCA has the edge. If you need speed and scale, EXESS wins.

## Accessibility & Cloud

Both EXESS and ORCA are free for academic use. ORCA requires local installation and command-line operation, while EXESS is available through a web browser with zero setup.

ORCA is closed-source despite being free, meaning you cannot inspect or modify the code. EXESS is also proprietary, but its cloud delivery model means you never need to manage binaries or dependencies.

## When to Choose EXESS

- You need GPU-accelerated calculations
- Your systems exceed a few hundred atoms
- You want browser-based access without installation
- Speed is more important than method breadth
- You need multi-node parallel scaling on GPU clusters
- You have an existing compute allocation with an HPC cluster and want to use it most efficiently

## When to Choose ORCA

- You need DLPNO-CCSD(T) or other coupled-cluster methods
- Your research requires multireference methods (CASSCF, NEVPT2)
- You need spectroscopy property calculations

## Frequently Asked Questions

### Is EXESS or ORCA better for DFT calculations?

For DFT on medium to large systems, EXESS is significantly faster due to GPU acceleration. ORCA offers broader functional coverage and is well-suited for smaller systems on CPU hardware. Both are free for academic use.

### Does ORCA support GPU acceleration?

No. ORCA is a CPU-only quantum chemistry program that parallelizes via MPI. It does not support GPU acceleration. If you need GPU-accelerated quantum chemistry, EXESS or TeraChem are your main options.

### Can I use EXESS and ORCA together?

Yes. Many researchers use EXESS for large-scale DFT production calculations and ORCA for high-accuracy benchmarks with DLPNO-CCSD(T). The two tools are complementary for different parts of a research workflow.

### Is ORCA truly free?

ORCA is free for academic and educational use. Commercial use requires a paid license. Note that ORCA is closed-source — you receive binaries but cannot access or modify the source code. EXESS is similarly free for academic use.


---

# EXESS vs Q-Chem

> Compare EXESS and Q-Chem for quantum chemistry. GPU performance, excited-state methods, pricing, and system size compared side-by-side.

URL: https://exess.qdx.co/comparisons/exess-vs-qchem

Q-Chem is a versatile commercial quantum chemistry package known for its excellent excited-state methods, including EOM-CCSD and ADC, and its IQmol graphical interface. It's a strong choice for photochemistry and spectroscopy research.

EXESS offers a fundamentally different value proposition: GPU-native performance for large-scale ground-state calculations at zero cost for academics. The choice depends on whether you need method breadth or computational throughput.

## Head-to-Head Comparison

| Feature | EXESS | Q-Chem |
| --- | --- | --- |
| Pricing | Pay-as-you-go (compute included) | License (~$2,000–$50,000+, excl. compute) |
| Free Academic Access | Yes | Free license on HPC facilities |
| GPU Support | Native GPU (CUDA + HIP) | Partial (TeraChem engine) |
| Methods | HF, DFT, RI-MP2, MBE | HF, DFT, EOM-CC, ADC, TDDFT |
| Hardware | GPU clusters (NVIDIA + AMD) | CPU (some GPU) |
| Cloud Availability | Yes — browser-based | IQmol GUI only |
| Parallel Scaling | Multi-node GPU (world first exascale calculation) | MPI multi-node |
| License | Proprietary (free academic) | Commercial |

## Performance & Scale

Q-Chem is primarily CPU-based, with some GPU capability through its integrated TeraChem engine. EXESS is fully GPU-native, delivering substantially faster DFT and HF calculations for systems of any size.

For large molecular systems (>500 atoms), EXESS's multi-node GPU architecture handles calculations that would be impractical in Q-Chem. Q-Chem's MPI parallelism scales well on CPU clusters but cannot match GPU throughput.

## Excited States & Specialized Methods

Q-Chem's standout feature is its excited-state method portfolio: EOM-CCSD, ADC(2), ADC(3), TDDFT, and various multi-reference approaches. For photochemistry, excited-state dynamics, and spectroscopy, Q-Chem is hard to beat.

EXESS focuses on ground-state methods (HF, DFT, RI-MP2, MBE) with maximum GPU performance. If your primary workflow involves ground-state energies, geometries, and large-scale DFT, EXESS is more efficient. If you need excited states, Q-Chem has the methods.

## Pricing & Access

Q-Chem licenses range from ~$2,000 to $50,000+, excluding compute costs.

EXESS is free for academic use and available via web browser. No license negotiation, IT setup, or hardware procurement required — just a browser and an internet connection.

## When to Choose EXESS

- You need fast ground-state DFT/HF on large systems
- You need browser-based cloud access
- Your focus is molecular energies and geometries, not excited states
- You have an existing compute allocation with an HPC cluster and want to use it most efficiently
- Speed is important

## When to Choose Q-Chem

- You need EOM-CCSD, ADC, or advanced excited-state methods
- Your research focuses on photochemistry or spectroscopy
- You need specialized analysis tools (CDA, EDA, etc.)

## Frequently Asked Questions

### Is EXESS faster than Q-Chem for DFT?

Yes. EXESS's GPU-native architecture delivers significant speedups over Q-Chem's CPU-based DFT for systems of all sizes. For large systems (>500 atoms), EXESS can be orders of magnitude faster due to multi-node GPU scaling.

### Does Q-Chem support GPU calculations?

Q-Chem has partial GPU support through an integrated TeraChem engine for certain DFT and HF calculations. However, most Q-Chem methods run on CPU. EXESS runs all its methods natively on GPU.

### How much does Q-Chem cost?

Q-Chem licenses range from approximately $2,000 to $50,000+ depending on license type and configuration, excluding compute costs. EXESS is free for academic use.

### Can EXESS do excited-state calculations like Q-Chem?

No. EXESS currently focuses on ground-state methods (HF, DFT, RI-MP2, MBE). For excited-state calculations (EOM-CCSD, TDDFT, ADC), Q-Chem or ORCA would be more appropriate choices.


---

# EXESS vs TeraChem

> Compare EXESS and TeraChem for GPU-accelerated quantum chemistry. Performance, multi-GPU scaling, pricing, AMD support, and cloud access compared.

URL: https://exess.qdx.co/comparisons/exess-vs-terachem

TeraChem pioneered GPU-accelerated quantum chemistry and remains a strong choice for researchers with NVIDIA hardware. Its CASSCF and TDDFT implementations are well-regarded for excited-state work on GPUs.

EXESS builds on the GPU-first philosophy but extends it further: support for both NVIDIA and AMD GPUs, multi-node GPU scaling, larger system sizes, and free academic access via the cloud.

## Head-to-Head Comparison

| Feature | EXESS | TeraChem |
| --- | --- | --- |
| Pricing | Pay-as-you-go (compute included) | License (contact directly for pricing) |
| Free Academic Access | Yes | No |
| GPU Support | Native GPU (CUDA + HIP) | Native GPU (CUDA) |
| Methods | HF, DFT, RI-MP2, MBE | HF, DFT, CASSCF, TDDFT |
| Hardware | GPU clusters (NVIDIA + AMD) | NVIDIA GPU |
| Cloud Availability | Yes — browser-based | TeraChem Cloud (limited) |
| Parallel Scaling | Multi-node GPU (world first exascale calculation) | Multi-GPU single node |
| License | Proprietary (free academic) | Commercial |

## GPU Architecture & Hardware

Both EXESS and TeraChem are GPU-native codes, but they differ in hardware support and scaling. TeraChem supports NVIDIA CUDA GPUs and can use multiple GPUs on a single node. EXESS supports both NVIDIA (CUDA) and AMD (HIP) GPUs and scales across multi-node GPU clusters.

## Pricing & Access

TeraChem requires a commercial license, though no public price estimate is available.

EXESS is free for academic use and runs in the cloud — no GPU hardware purchase, CUDA driver management, or IT support needed. This makes high-performance GPU quantum chemistry accessible to groups without dedicated GPU infrastructure.

## When to Choose EXESS

- You need support for AMD GPUs (HIP)
- Your systems exceed 2,000 atoms
- You need multi-node GPU scaling
- You want cloud-based access without managing GPU hardware
- You want free academic access
- You have an existing compute allocation with an HPC cluster and want to use it most efficiently
- Speed is important

## When to Choose TeraChem

- You need GPU-accelerated CASSCF or TDDFT
- You're working on single-node multi-GPU setups with NVIDIA hardware

## Frequently Asked Questions

### How does EXESS compare to TeraChem for GPU performance?

Both are GPU-native codes with strong single-GPU performance. EXESS differentiates with multi-node GPU scaling (enabling larger calculations) and AMD GPU support via HIP.

### Does EXESS support AMD GPUs like TeraChem does not?

Yes. EXESS supports both NVIDIA GPUs (via CUDA) and AMD GPUs (via HIP). TeraChem only supports NVIDIA CUDA GPUs. This makes EXESS the better choice for institutions with AMD Instinct hardware.

### How much does TeraChem cost compared to EXESS?

TeraChem requires a commercial license, though no public price estimate is available. EXESS is free for academic use and available via the cloud, requiring no local GPU hardware or license fees.

### Which is better for ab initio molecular dynamics: EXESS or TeraChem?

TeraChem has established AIMD capabilities with GPU acceleration. EXESS has performed AIMD on systems with hundreds of thousands of atoms and supports single-point energy calculations, geometry optimizations, large-scale MBE workflows, and GPU-accelerated AIMD at unprecedented scale.


---

# Free Quantum Chemistry Software

> Compare free quantum chemistry software: EXESS, ORCA, NWChem, CP2K, Psi4, and GAMESS. GPU support, methods, system size, and ease of use compared for academic research.

URL: https://exess.qdx.co/comparisons/free-quantum-chemistry-software

Several excellent quantum chemistry packages are available free for academic use, and a growing number are fully open-source.

This guide compares the leading free quantum chemistry options — from GPU-accelerated cloud platforms to established open-source codes — to help you find the right tool for your research.

## Free Software Comparison Table

| Feature | EXESS | ORCA | NWChem | CP2K | Psi4 | GAMESS |
| --- | --- | --- | --- | --- | --- | --- |
| Pricing | Pay-as-you-go (compute included) | License (contact directly for pricing) | Free (open-source) | Free (open-source) | Free (open-source) | Free (registration required) |
| Free Academic Access | Yes | Yes | Yes | Yes | Yes | Yes |
| GPU Support | Native GPU (CUDA + HIP) | No | Experimental | GPU via dbcsr/COSMA | Limited (gpu_dfcc plugin) | Limited (LibCChem) |
| Methods | HF, DFT, RI-MP2, MBE | HF, DFT, MP2, CCSD(T), DLPNO | HF, DFT, MP2, CCSD(T), TDDFT | DFT, MP2, GW, QM/MM | HF, DFT, MP2, CCSD(T), SAPT | HF, DFT, MP2, MCSCF, CI |
| Hardware | GPU clusters (NVIDIA + AMD) | CPU only | CPU (experimental GPU) | CPU + GPU (NVIDIA) | CPU (some GPU plugins) | CPU (limited GPU) |
| Cloud Availability | Yes — browser-based | No | No | No | No | No |
| Parallel Scaling | Multi-node GPU (world first exascale calculation) | MPI multi-node | Strong MPI scaling | Strong MPI+OpenMP | Threaded + MPI | DDI parallel |
| License | Proprietary (free academic) | Free academic (closed-source) | ECL 2.0 (open-source) | GPL v2 (open-source) | LGPL v3 (open-source) | Free academic (source available) |

## Free vs Open-Source

An important distinction: "free" and "open-source" are not the same. EXESS and ORCA are free for academic use but proprietary (closed-source). NWChem, CP2K, Psi4, and GAMESS provide source code access under various licenses.

Open-source codes let you inspect, modify, and contribute to the software. Free-but-closed codes offer convenience and polish without the ability to examine the implementation.

## GPU Support in Free Software

EXESS has full native GPU support (CUDA + HIP). CP2K has GPU support for some operations via dbcsr/COSMA libraries. NWChem has experimental GPU code. Psi4 has a gpu_dfcc plugin for coupled-cluster.

If GPU acceleration is important to your workflow, EXESS is currently the only free option that delivers production-quality GPU performance across all its methods.

## Installation & Ease of Use

Ease of setup varies dramatically. EXESS requires no installation (browser-based). Psi4 installs via conda in minutes. ORCA provides pre-built binaries. At the other extreme, CP2K and NWChem often require building from source with complex dependency chains.

For students and researchers without dedicated IT support, EXESS and Psi4 offer the lowest barrier to entry. ORCA is also straightforward if you're comfortable with command-line tools.

## When to Choose EXESS

- You need GPU-accelerated calculations
- You want browser-based access with zero setup
- Your systems exceed a few hundred atoms
- You need production-quality performance without license fees
- You have an existing compute allocation with an HPC cluster and want to use it most efficiently
- Speed is important

## Frequently Asked Questions

### What is the best free quantum chemistry software?

It depends on your needs. For GPU-accelerated calculations on large systems, EXESS offers the best performance, free for academics. For broad method coverage without GPU, ORCA is excellent. For open-source flexibility and method development, Psi4 (wavefunction methods) or CP2K (periodic/QM-MM) are strong choices.

### Is Gaussian free for academic use?

No. Gaussian requires a paid license for all users, including academics. Free alternatives include EXESS, ORCA (both free for academic use), and open-source options like Psi4, NWChem, CP2K, and GAMESS.

### Which free quantum chemistry software supports GPU?

EXESS is free for academics and has full native GPU support (NVIDIA CUDA + AMD HIP). CP2K has partial GPU support for certain operations. NWChem has experimental GPU code. Most other free options (ORCA, Psi4, GAMESS) are CPU-only.