# 3.3.13. GUGA — A Configuration Interaction Coupling Coefficients Program¶

Several of the Configuration Interaction (CI) modules in Molcas use the GUGA module to compute the CI coupling coefficients. We continue our water calculations using the input file shown in the input below. 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.

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
Title= GUGA for C2v Water
Electrons= 8; Spin= 3
Inactive= 1 0 0 0; Active= 2 2 0 1
CIAll= 1


The keywords CIALl and REFErence are mutually exclusive. We specify CIALl which will calculate the energy using all possible references functions that can be constructed using the input set of occupation numbers of the active orbitals regardless of the spin coupling (all configurations used to build the corresponding CASSCF wave function). 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 functions and the ACTIve keyword specifies the orbitals that may have varying occupations in all references. The selection of INACtive orbitals in the input above is forcing the bonding sp hybrid orbital to remain fully occupied in all reference states.

## 3.3.13.1. 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.