CC4S interface

CP2K can export a converged restricted Hartree–Fock reference for MP2, CCSD and CCSD(T) calculations with CC4S. CP2K writes canonical orbital energies, a Gaussian RI Coulomb vertex and a CC4S input file. Run CC4S separately to calculate correlation energies. CP2K does not need to be linked against CC4S.

The interface has been tested with CC4S v1.1, revision 8204fb21. The underlying methods are described in the RI-HFX paper and the auxiliary-field representation paper.

Reference calculation

Use GLOBAL/RUN_TYPE ENERGY and a converged, restricted closed-shell HF calculation with integer occupations, at least one occupied and one virtual orbital, and a positive HOMO–LUMO gap. Set XC_FUNCTIONAL NONE, use exactly one HF section with FRACTION 1, and select HF/RI/RI_FLAVOR MO. Supply a suitable Gaussian fitting basis through BASIS_SET RI_HFX for each atomic kind. These settings belong to RI-HFX; the WF_CORRELATION integral settings do not configure this exporter. ADMM and smearing are unsupported. ENERGY_FORCE is also accepted, but any forces are HF forces.

Enable export

Add the following inside FORCE_EVAL/DFT/PRINT to enable export. The filename prefix and CC4S-specific keywords are shown with their default values:

&CC4S ON
  FILENAME cc4s
  METHOD CCSD
  N_FROZEN_CORE 0
  MAX_ITER 100
  ENERGY_CONVERGENCE 1e-7
  AMPLITUDES_CONVERGENCE 1e-6
  MAX_MEMORY 1024
&END CC4S

Keyword	Meaning
FILENAME	Common prefix for the generated files, using standard CP2K print-key naming.
METHOD	MP2 generates MP2; CCSD adds CCSD; CCSD_T also adds perturbative triples.
N_FROZEN_CORE	Number of lowest occupied spatial orbitals excluded from correlation; must leave at least one occupied orbital. With pseudopotentials, excludes additional explicitly treated orbitals.
MAX_ITER	Maximum CC4S coupled-cluster iterations.
ENERGY_CONVERGENCE	CC4S energy convergence threshold, in Hartree.
AMPLITUDES_CONVERGENCE	CC4S amplitude convergence threshold.
MAX_MEMORY	Estimated export workspace limit, in MB per MPI rank; excludes existing SCF storage and CC4S memory.

Export is OFF by default. ON enables it independently of GLOBAL/PRINT_LEVEL; standard print levels are also accepted. The iteration and convergence settings above control CC4S, independently of CP2K’s SCF and integral thresholds. Generic EACH and ADD_LAST controls need not be specified; they select output only when the post-SCF exporter is called.

Complete small input examples are provided in tests/QS/regtest-cc4s: h2o.inp with its BASIS_CC4S file, and he-gamma.inp. These are regression examples, not numerically converged production settings.

Boundary conditions and interactions

System	CELL and POISSON periodicity	HF potential	RI metric
Isolated molecule	NONE in both	COULOMB	COULOMB
Neutral three-dimensional Γ-point cell	XYZ in both	TRUNCATED	HFX

Periodic export supports Γ-point calculations only. CUTOFF_RADIUS must be positive and smaller than half the minimum perpendicular spacing between opposite cell faces.

Exported files

Export occurs once after converged SCF in a single-point calculation. Geometry optimization and MD are unsupported.

With PROJECT water and the defaults above, the files in the CP2K working directory are:

File	Contents
water-cc4s-EigenEnergies.yaml	Tensor header, Fermi level and complete orbital-energy list in metaData.energies
water-cc4s-EigenEnergies.elements	Plain-text orbital energies in Hartree
water-cc4s-CoulombVertex.yaml	Tensor header, dimensions and interaction metadata
water-cc4s-CoulombVertex.elements	Raw binary real 64-bit vertex elements, without Fortran record markers
water-cc4s.in	CC4S algorithm sequence and solver settings

With PROJECT water:

• FILENAME sample produces water-sample-EigenEnergies.yaml, water-sample-CoulombVertex.yaml and water-sample.in, with matching .elements files.

• FILENAME ./sample produces sample-EigenEnergies.yaml, sample-CoulombVertex.yaml and sample.in, with matching .elements files and no project prefix.

Paths may refer to existing directories; CP2K does not create them. Exact-name (=name) and stdout forms are unsupported because this export writes multiple distinct files. The generated input references the matching tensor filenames; run CC4S from the directory containing these files. Standard iteration suffixes, if selected, apply to the whole file set.

The vertex has index order (auxiliary, orbital, orbital), with the auxiliary index varying fastest. Its units are the square root of Hartree. The headers use tensor format version 100, Real64 and unit: 1.0. No Fock-matrix file is written. See the CC4S CoulombVertex specification for the tensor convention; CP2K supplies Gaussian fitting factors rather than the plane-wave construction discussed there.