It is available for the elements H through Cl. The method used in Gaussian 09 is based on reference. Īpproximate spin orbit coupling between two spin states can be computed during CASSCF calculations by including the SpinOrbit option. State-averaged CASSCF calculations may be performed using the StateAverage and NRoot options to specify the states to be used.Ĭonical intersections and avoided crossings may be computed by including Opt=Conical in the route section of a CASSCF job (see the examples). Note that a value of 1 specifies the ground state, not the first excited state (in contrast to usage with the CIS or TD keywords). Ĭalculations on excited states of molecular systems may be requested using the NRoot option. VARIATIONSĪn MP2-level electron correlation correction to the CASSCF energy may be computed during a CASSCF calculation by specifying the MP2 keyword in addition to CASSCF within the route section. ![]() Example applications are discussed in references. We strongly recommend that you study the cited references before attempting to run production CASSCF calculations (this is especially true for CASSCF MP2). Note: CASSCF is a powerful but advanced method with many subtleties. See Efficiency Considerations for a discussion of CASSCF calculation performance. See reference for a detailed discussion on the choice of an active space. Use #P in the route section to include the final eigenvalues and eigenvectors in addition to the energy and one-electron density matrix in the CASSCF output.Ī brief overview of the CASSCF method is given in chapter 9 (exercises 5 and 6) and appendix A of Exploring Chemistry with Electronic Structure Methods, 2nd ed. You may also choose to view the orbitals in a visualization package such as GaussView. Alternatively, you may use Pop=NBOSave to save the NBOs, which are often the best choice for starting CAS orbitals. ![]() You need to include Pop=Regular in the route section of the preliminary job in order to include the orbital coefficient information in the output (use Pop=Full for cases where you need to examine more than just the few lowest virtual orbitals). Alternatively, a full Hartree-Fock single point calculation may be done, and the subsequent job will include Guess=(Read,Permute) in order to retrieve and then modify the computed initial guess from the checkpoint file. A prior run with Guess=Only can be used to quickly determine the orbital symmetries (see the first example below). Normally, Guess=Alter or Guess=Permute is necessary to ensure that the orbitals which are selected involve the electrons of interest and that they are correlated correctly. In Gaussian 09, algorithmic improvements make an active space of up to about 14 orbitals feasible. ![]() Similarly, a 4 electron, 6 orbital CAS on a triplet would include the highest 3 occupied orbitals (one of which is doubly occupied and two singly occupied in the guess determinant) and the lowest 3 virtual orbitals. Since 2 occupied orbitals were included, the lowest 4 virtual orbitals would become part of the active space. Thus, the 2 highest occupied MOs would be included.Įnough virtual orbitals to make a total of 6 orbitals. Thus, for a 4-electron, 6-orbital CAS-specified as CASSCF(4,6)-on a closed-shell system, the active space would consist of:Įnough occupied orbitals from the guess to provide 4 electrons. Note that options may be interspersed with N and M in any order.īy default, the active space is defined assuming that the electrons come from the highest occupied orbitals in the initial guess determinant and that the remaining orbitals required for the active space come from the lowest virtuals of the initial guess. ![]() The number of electrons ( N) and the number of orbitals ( M) in the active space for a CASSCF must be specified following the keyword: CASSCF( N, M). An MC-SCF calculation is a combination of an SCF computation with a full CI involving a subset of the orbitals this subset is known as the active space. This method keyword requests a Complete Active Space Multiconfiguration SCF (MC-SCF).
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