Q-Chem 5.2 features a new package libnoci dedicated to running NOCI calculations, and accessed using USE_LIBNOCI = TRUE. The libnoci implementation introduces flexible job control, whereby the method used to generate multiple states for the NOCI basis can be defined by the user. Initially, a set of reference determinants must be chosen, either using a single SCF calculation or by reading sets or orbital coefficients from file as requested by the $rem variable NOCI_REFGEN. From these reference states, multiple non-orthogonal states are then created either using SCF metadynamics, or as excitations from the reference determinants, as requested by the $rem variable NOCI_DETGEN. When generating multiple using excitations within an active space, the active orbitals are controlled using the $active_orbitals input section. For example:
$active_orbitals 1 4 11 14 $end
The orbitals are offset by the total number of molecular orbitals (e.g. the above example selects orbitals 1 and 4, along with orbitals 1 and 4, with a total of 10 MOs including occupied and virtual orbitals). The resulting multiple determinants are then individually relaxed at the SCF level, unless this is explicitly skipped using SKIP_SCFMAN = TRUE. Finally, NOCI is run using the full set of multiple determinants identified.
The libnoci package also incorporates compatibility with the new SCF metadynamics implementation (see Section 4.9.3), as well as the holomorphic Hartree–Fock approach which allows multiple Hartree–Fock solutions to be continued across all geometries (see Section. 4.9.4).
$molecule 0 1 H 0.0000000 0.0000000 0.0000000 H 0.0000000 0.0000000 4.0000000 $end $rem EXCHANGE hf CORRELATION noci UNRESTRICTED true BASIS sto-3g SCF_CONVERGENCE 10 MAX_SCF_CYCLES 1000 MOM_START 1 USE_LIBNOCI true SCF_SAVEMINIMA 4 SCF_MINFIND_RANDOMMIXING 30000 SCF_MINFIND_MIXMETHOD 1 NOCI_REFGEN 0 NOCI_DETGEN 3 NOCI_NEIGVAL 4 $end