Subsections

• • 3.10.0.1 GUGA Output

3.10 GUGA -- A Configuration Interaction Coupling Coefficients Program

Several of the Configuration Interaction (CI) modules in MOLCAS use the guga module to compute the coupling coefficients. We continue our water calculations using the input file shown in Figure 3.10. The TITLe keyword behaves in a similar fashion as described in previous modules. There are several compulsory keywords of the guga module. The number of electrons to be correlated is specified using the ELECtrons keyword. We are correlating the valence electrons. The spin state is specified using the SPIN keyword. The SYMMetry keyword specifies the order of the point group used in the calculation. The C$_{2v}$ point group has order four. The SYMMetry keyword is different than in the rasscf module where it specifies the symmetry of the wave function requested, not the order of the point group.

Figure 3.10. Sample input requesting the the guga module to calculate the coupling coefficients for neutral triplet water in C$_{2v}$ symmetry with six electrons in the active space.

 &GUGA &END
Title
  GUGA for C2v Water
Electrons
 8
Spin
 3
Symmetry
 4
CIAll
 1
Inactive
 1 0 0 0
Active
 2 2 0 1
End of Input

The keywords CIALl and REFErence are mutually exclusive. We specify CIALl which will calculate the energy using all possible references as specified by the input set of occupation numbers of the active orbitals regardless of the spin coupling. Specific selected references can be chosen using the REFErence keyword. Either the ACTIve or INACtive keyword should be used for a meaningful calculation. The default for both keywords is zero for all symmetries. These keywords function in a similar fashion to these in the RASSCF program module. The INACtive keyword specifies the orbitals that are fully occupied in each symmetry in all the reference states and the ACTIve keyword specifies the orbitals that may have varying occupations in all references. The selection of INACtive orbitals in Figure 3.10 is forcing the bonding $sp$ hybrid orbital to remain fully occupied in all reference states.

3.10.0.1 GUGA Output

The GUGA section of the output lists the possible configurations in the active space. There are nine possible triplet configurations of six electrons in five orbitals. Apart from the various types of orbital in each symmetry the GUGA section of the output also gives the number of states that will coupled with various states. There are no input files for the GUGA module but the calculated coupling coefficients are stored in CIGUGA.