# 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.