A combined freeze‐and‐cut strategy for the description of large molecular systems using a localized orbitals approach

A technique to reduce the computational effort in calculating ab initio energies using a localized orbitals approach is presented. By exploiting freeze strategy at the self‐consistent field (SCF) level and a cut of the unneeded atomic orbitals, it is possible to perform a localized complete active space (CAS‐SCF) calculation on a reduced system. This will open the possibility to perform ab initio treatments on very large molecular systems, provided that the chemically important phenomena happen in a localized zone of the molecule. Two test cases are discussed, to illustrate the performance of the method: the cis–trans interconversion curves for the (7Z)‐13 ammoniotridec‐7‐enoate, which demonstrates the ability of the method to reproduce the interactions between charged groups; and the cisoid–transoid energy barrier for the aldehydic group in the C 13 polyenal molecule. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1042–1051, 2005