The CASSCF method is multi-configurational while standard CI is not
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Reference literature establishes that the CASSCF method functions as a multi-configurational self-consistent field approach where both expansion coefficients and molecular orbitals are optimized, whereas standard configuration interaction uses fixed orbitals derived from a single reference.
# How does optimizing the molecular orbital coefficients in CASSCF improve its multi-reference capabilties?
Tags: quantum-chemistry, theoretical-chemistry, ab-initio, multi-reference
- Score: 12
- Views: 2283
- Answers: 2
- Answered: yes
- Asked by: Yoda (4853 rep)
- Asked: 2017-05-13
- Edited: 2018-11-16
- Site: chemistry
## Question
The complete active space self-consistent field (CASSCF) method is a special kind of multi-configurational SCF procedure, attempting to combine Hartree-Fock (HF) and configuration interaction (CI). In HF, the molecular orbital (MO) expansion coefficients are varied during the procedure, but the wave function contains just one Slater determinant. In CI, the multielectron determinant expansion coeffcients are varied, but the MO's used to build the excited Slater determinants are taken from HF, and are not varied during the energy-minimization.
During CASSCF, on the other hand, all MO expansion coefficients are optimized during the energy-minimization procedure, in addition to the multielectron expansion coefficients.
CI is able to recover the dynamical correlation of the system (configuration with large weight, many configurations with small weight