.. index:: single: Program; GUGA single: GUGA .. _TUT\:sec\:guga: :program:`GUGA` --- A Configuration Interaction Coupling Coefficients Program ============================================================================= Several of the Configuration Interaction (CI) modules in |molcas| use the :program:`guga` module to compute the CI coupling coefficients. We continue our water calculations using the input file shown in the input below. The :kword:`TITLe` keyword behaves in a similar fashion as described in previous modules. There are several compulsory keywords of the :program:`guga` module. The number of electrons to be correlated is specified using the :kword:`ELECtrons` keyword. We are correlating the valence electrons. The spin state is specified using the :kword:`SPIN` keyword. .. index:: single: GUGA; Input Sample input requesting the the GUGA module to calculate the coupling coefficients for neutral triplet water in :math:`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 :kword:`CIALl` and :kword:`REFErence` are mutually exclusive. We specify :kword:`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 :kword:`REFErence` keyword. Either the :kword:`ACTIve` or :kword:`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 :program:`RASSCF` program module. The :kword:`INACtive` keyword specifies the orbitals that are fully occupied in each symmetry in all the reference functions and the :kword:`ACTIve` keyword specifies the orbitals that may have varying occupations in all references. The selection of :kword:`INACtive` orbitals in the input above is forcing the bonding sp hybrid orbital to remain fully occupied in all reference states. :program:`GUGA` Output ---------------------- The :program:`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 :program:`GUGA` section of the output also gives the number of states that will coupled with various states. There are no input files for the :program:`GUGA` module but the calculated coupling coefficients are stored in :file:`CIGUGA`.