Electronic Structure Methods

Supported methods at a glance

Basis sets (summary)

Family	Examples	Notes
Pople	6-31G, 6-31G*, 6-31++G**, 6-311G**	Common split-valence sets.
Dunning	cc-pVDZ, cc-pVTZ, cc-pVQZ, aug-cc-pVTZ	RIFIT auxiliary variants available.
def2	def2-SVP, def2-TZVP, def2-TZVPP, def2-QZVP	RIFIT auxiliary variants available.
STO-nG	STO-2G, STO-3G, STO-6G	Minimal basis sets.
PCSeg	PCSeg-0, PCSeg-1	Segmented basis sets.
X2C	x2c-SVPall	Relativistic all-electron basis.

For the full list, see the reference page.

DFT functionals

EXESS uses LibXC through ExchCXX. Any LibXC functional name accepted by the build can be requested directly; invalid names cause EXESS to error and print the available functionals.

EXESS also supports custom linear combinations in the ExchCXX syntax, for example: 0.2*gga_x_pbe+0.8*gga_x_b88+0.2*hf.

Built-in aliases with EXESS-defined coefficients include:

• SVWN5 (expanded as LDA_X+LDA_C_VWN)

• B2PLYP

• REVDSD-PBEP86-D4

• REVDSD-PBEP86-D4(NOFC)

Correlated methods

• RI-MP2 (RestrictedRIMP2) is the supported MP2 implementation.

• RestrictedRICCSD is available in the method list; check deployment support before relying on it.

Dynamics capabilities

• Born-Oppenheimer AIMD with a Verlet integrator (microcanonical only).

• QMMM dynamics with NVT or NPT control via qmmm.temperature_kelvin and qmmm.pressure_atm.

• Periodic boundary conditions.

• Water-only classical solvent support.

How to choose settings (practical guidance)

Full-system vs fragmentation

• Use full-system calculations for small to medium systems that fit on a single node.

• Use fragmentation for larger systems or when you need multi-node scaling.

• Accuracy improves with higher-order MBE truncation (dimer < trimer < tetramer), but cost grows combinatorially.

Building fragments and cutoffs

• Provide fragments explicitly as lists of atom indices.

• Use connectivity if covalent bonds are cut so EXESS can apply hydrogen capping correctly.

• Use cutoffs to limit long-range n-mers and control cost; keep dimer > trimer > tetramer when using multiple cutoffs.

• Use cutoff_type to choose centroid vs closest-pair distances depending on your system.

RI and basis set strategy

• RI is required for MP2 (only RI-MP2 is implemented).

• RI-HF/RI-MP2 allow higher angular momentum basis sets than non-RI HF.

• Always supply a matching aux_basis when using RI.

DFT strategy

• Start with ks_dft.method = "BatchDense" and the default grid.

• Use GauXC only when you need it for compatibility or benchmarking.

Gradients, dynamics, and optimization

• Dynamics and optimization require gradients. Analytical gradients are only available for restricted HF and restricted RI-MP2; use numerical derivatives otherwise.

• AIMD is microcanonical only, so choose timestep and total steps accordingly.

• For thermostatted or barostatted dynamics, use QMMM and set keywords.regions with qmmm.temperature_kelvin (and optional qmmm.pressure_atm).

• Geometry optimization defaults to internal coordinates; override only if you understand the tradeoffs.

GPU scaling and system settings

• Fragmentation is the scaling path for multi-node runs.

• Tune system.teams_per_node and system.gpus_per_team to control how fragments map to GPUs.

• Use system.max_gpu_memory_mb to cap GPU memory if needed (be conservative; requesting more than available will crash).

Start here: first calculation

Minimal RHF energy input (single topology):

{
  "topologies": [
    { "xyz": "/path/to/water.xyz" }
  ],
  "driver": "Energy",
  "model": {
    "method": "RestrictedHF",
    "basis": "cc-pVDZ",
    "aux_basis": "cc-pVDZ-RIFIT"
  },
  "keywords": {}
}

Then run with the EXESS executable or the rush-py client. See the running guide for CLI and rush-py commands, and the examples page for progressively more advanced workflows.

Hartree-Fock

EXESS supports RHF and UHF energies. Analytical gradients are available for restricted HF but not UHF. Spherical basis functions (force_cartesian_basis_sets=false) are only supported for Energy calculations.

RI-HF and RI-MP2

RI is the main path to scalable HF and MP2 in EXESS:

• RI-HF is enabled by setting scf.fock_build_type = "RI" with method = "RestrictedHF".

• RI-MP2 is the supported MP2 implementation (method = "RestrictedRIMP2"). MP2 runs are RI by design.

RI reduces computational cost but increases memory use because integrals are stored. RI-HF, RI-MP2, and RI-CCSD require an auxiliary basis set, and double-hybrid KSDFT functionals require one as well. In return, RI-HF/RI-MP2 support higher angular momentum basis sets (up to F in the primary basis and G in the auxiliary basis).

KSDFT

Restricted KSDFT is supported via LibXC functionals (DFT support must be compiled into EXESS). Unrestricted DFT is not implemented. Analytical gradients are not available for KSDFT; use numerical derivatives when needed.

Method support summary (analytical gradients only):

Calculation	RHF	UHF	RI-HF	RI-MP2	RestrictedKSDFT
Energy	yes	yes	yes	yes	yes
Analytical gradient	yes	no	yes	yes	no

Fragmentation theory (MBE)

EXESS implements a Many-Body Expansion (MBE) for non-covalent and covalent systems, with truncation up to fourth order (tetramers). The number of n-mers grows combinatorially with fragment count:

C(n, k) = n! / (k! * (n - k)!)

Covalent fragmentation uses hydrogen capping for broken single bonds. If you cut covalent bonds, provide connectivity so EXESS can cap and restore bonds correctly.

For background and accuracy considerations, see https://doi.org/10.1063/1.5126216 and https://doi.org/10.1021/cr200093j.

Gradients, dynamics, and optimization

• Geometry optimization is supported for RHF, RI-HF, and RI-MP2.

• Born-Oppenheimer AIMD uses a Verlet integrator and is microcanonical only (no thermostats or barostats).

• QMMM dynamics support NVT/NPT via the qmmm block (temperature and optional pressure), with MM handled through OpenMM and ML via AIMNet.

• Dynamics can be combined with fragmentation for large systems.

• Periodic boundary conditions and water-only classical solvent support are available in AIMD workflows.

Limitations

EXESS is strong when you need GPU performance and fragmentation at scale. It is not a general-purpose electronic structure code with every method or model.

Limitations:

• Unrestricted KSDFT is not implemented.

• Unrestricted RI-MP2 is not supported (use RestrictedRIMP2).

• The UM09 (MMD) Fock build path is not defined for UHF when using spherical basis functions.

• Analytical gradients are limited to restricted HF and restricted RI-MP2.

• Hydrogen capping is the only supported covalent bond-breaking scheme.

• Plane-wave basis sets are not supported.

• Fully ab initio AIMD is microcanonical; thermostatted/barostatted dynamics require QMMM.

Hardware guidance and known issues are listed in the reference page.