OpenMolcas is a widely used quantum chemistry package which specializes in multiconfigurational approaches. Here, we focus on how to use the software in tandem with OpenCAP to perform calculations on resonances.

System

OpenCAP can extract the basis set and geometry from two types of files generated by OpenMolcas: Molden, and rassi.h5. Set the "molecule" keyword to either "molden" or "molcas_rassi", and then set "basis_file" to the properly formatted molden/rassi.h5 file. The geometry and basis set can also be specified manually by setting the "molecule" keyword to inline, and setting "basis_file" to a basis set file specified in Psi4 format. Basis sets can be downloaded from the MolSSI and modified to suit your purposes.

Relevant keywords

Keyword	Valid options	Description
molecule	molden,rassi_h5,inline,qchem_fchk	Specifies which format to read the molecular geometry. If "inline" is chosen, the "$geometry" section is also required.
basis_file	path to basis file	Specifies the path to the basis file. When "molecule" is set to "molden","rassi_h5", or "qchem_fchk", this keyword should be set to a path to a file of the specified type. When "molecule" is set to "inline", this keyword should be set to a path to a basis set file formatted in "Psi4" style.

Projected CAP

The key to the OpenMolcas interface is the RAS State Interaction program (RASSI). As stated in the OpenMolcas reference, RASSI "computes matrix elements of the Hamiltonian and other operators in a wave function basis, which consists of individually optimized CI expansions from the RASSCF program". Of crucial importance to us is that RASSI is able to compute and export one particle transition density matrices between each pair of states. When the 'TRD1' keyword is activated, these densities are exported to an HDF5 file titled $JOBNAME.rassi.h5. When using XMS-CASPT2, RMS-CASPT2, or other variants which utilize rotated CASSCF wave functions, the CAP matrix will be rotated into the new basis using the rotation matrix. All MS-CASPT2 variants are specified as "MS-CASPT2" in the input.

 &RASSI
 TRD1

OpenCAP uses these densities to compute the CAP matrix elements in state basis, which is required to perform projected CAP calculations.

In addition to the one particle densities, OpenCAP also requires a zeroth order Hamiltonian. For a proper description of correlation, an effective Hamiltonian from a multi-state second order perturbation theory method is required. OpenCAP is capable of reading the effective Hamiltonian from OpenMolcas outputs for the following methods: MS-CASPT2,XMS-CASPT2,XDW-CASPT2,RMS-CASPT2,SC-NEVPT2,PC-NEVPT2.

Relevant keywords

Keyword	Type	Description
rassi_h5	string	Relative or absolute path to rassi.h5 file containing TDMs.
molcas_output	string	Relative or absolute path to OpenMolcas output file.
h0_file	string	Relative or absolute path to properly formatted zeroth order Hamiltonian file.

Example input

$system
molecule molden
basis_file xms.rasscf.molden
$end

$projected_cap
method ms-caspt2
package openmolcas
rassi_h5 xms.rassi.h5
molcas_output xms.out
nstates 10
CAP_TYPE box
CAP_X 2.76
CAP_Y 2.76
CAP_Z 4.88
Radial_precision  14
Angular_points    110
$end