# 3.3.13. MRCI — A Configuration Interaction Program¶

Multi Reference Single and Doubles Configuration Interaction (MR-SDCI) wave functions are produced by the MRCI program module in the Molcas codes. The SDCI keyword requests an ordinary Multi Reference Single and Doubles Configuration Interaction calculation. This is the default and is mutually exclusive with the ACPF keyword which requests an Average Coupled Pair Function calculation. The final keyword, ROOT, specifies the number of the CI root the calculation should compute. The second CI root is the first excited state and since the GUGA module has computed the coupling coefficients for a triplet state, the MRCI module will converge to the first excited triplet state.

## 3.3.13.1. MRCI Output¶

The MRCI section of the output lists the number of each type of orbital in each symmetry including pre-frozen orbitals that were frozen by the GUGA module. There is a list of the reference configurations with the inactive orbitals included. An empty orbital is listed as “0” and a doubly occupied as “3”. The spin of a singly occupied orbital by “1” (spin up) or “2” (spin down). The total number of configuration state functions (CSFs) is listed below the reference configurations.

Sample input requesting the the MRCI module to calculate the first excited MRCI energy for neutral triplet water in $$C_{2v}$$ symmetry with six electrons in the active space:

&MRCI
Title= MR-SDCI of 2nd CI root of C2v Water
SDCI; Root= 2


A listing of the possible CI roots is followed by the CI iteration and convergence information. The Davidson and ACPF corrections are included along with the important CSFs in the CI wave function. The molecular orbitals are listed near the end of the output.

There are four input files to the MRCI module; CIGUGA from GUGA, TRAONE and TRAINT from MOTRA and ONEINT from SEWARD. The orbitals are saved in CIORBnn where nn is the number of the CI root.