Capabilities and Limitations

Current capabilities:

EXESS capabilities will be outlined in three sets:

  • Fragmentation methods

  • Basic theories, such as Hartree-Fock, MP2, etc

  • General routines: Ab initio molecular dynamics, optimization procedures, etc

  • Properties: Mulliken charges, electrostatic potentials, etc.

Fragmentation methods

Fragmentation methods are key approach to perform accurate quantum chemistry simulations on large molecular systems. EXESS currently supports the Many Body Expansion (MBE) as the main fragmentation driver. The MBE algorithm in EXESS works for non- and covalently-bonded systems; for example: a water droplet with n waters, and a protein.

Currently, the MBE in EXESS supports truncation on up to the fourth order, i.e. calculating tetramers. The larger the truncation order (level) the more accurate the calculation will become. However, the number of polymers (dimers, trimers, tetramers) grows with respect to the number of fragments in the calculation. Y EXESS is able to perform a Many Body Expansion calculation on up to tetramers. The MBE implementation in EXESS can break single bonds using Hydrogen capping to account for loose charges. The number of fragments in a given calculation is given by the binomial coefficient:

\(C(n, k) = \binom{n}{k} = \frac{n!}{k!(n-k)!}\)

Therefore, a careful consideration of level of truncation and number of fragments needs to be established. Please refer to https://doi.org/10.1063/1.5126216 and https://doi.org/10.1021/cr200093j for thorough reviews on fragmentation methods.

Levels of theory

Calculation

RHF

UHF

RI-HF

RI-MP2

Energy

✓

✓

✓

✓

Gradient

✓

✓

✓

Fragmentation

✓

✓

✓

  • If gradients are enabled, fragmented gradients are also enabled

  • The RHF program will support basis sets that contain up to D functions.

  • The RI approximation for HF is only available for RHF

  • Spherical Gaussians are only supported for RHF-SCF at the moment

  • The RI-HF/MP2 programs will support basis sets that contain up to G functions in the primary basis set with their respective auxiliary

General routines

  • Optimization

  • Born-Oppenheimer Ab initio Molecular Dynamics (BO-AIMD)

Geometry Optimization

EXESS can use any of the RHF, RI-RHF, and RI-MP2 levels of theory to perform geometry optimization calculations. For an example calculation see Example geometry optimization calculation.

In geometry optimization calculations it is extremely important to be aware of the type of coordinates used in the procedure. The default is internal coordinates, which have been shown to be the best ones. See the manual on optimization for a deep dive on the keywords and their associated effects.

Born-Oppenheimer Ab initio MD

EXESS is able to harness the gradient implementations at the RHF, RI-HF, and RI-MP2 level of theory to perform ab initio molecular dynamics simulations using a Verlet integrator.

Currently, only the microcanonical ensemble is supported, no thermostats or barostats are included in EXESS. The ab initio MD routine is also interfaced with fragmentation, allowing for large molecular systems to be studied using AIMD.

Periodic boundary conditions and the ability to simulate solvent (water only) using classical force fields is also included.

Limitations

  • Support for up to D functions for non-RI Hartree-Fock

  • Support for up to G functions for RI based methods (RI-HF, RI-MP2)

  • Spherical Gaussians are only supported for non-RI HF-SCF calculations

  • Only hydrogen capping bond breaking is supported

  • Only support for non RI UHF

  • No plane wave basis sets supported