Molcas

The basic philosophy behind Molcas is to develop methods that will allow an accurate ab initio treatment of very general electronic structure problems for molecular systems in both ground and excited states. This is not an easy task. Our knowledge about how to obtain accurate properties for single reference dominated ground states is today well developed and MOLCAS contains a number of codes that can perform such calculations (MP2, CC, CPF, CCSD(T) etc.). All these methods treat the electron correlation starting from a single determinant (closed or open shell) reference state. Such codes are today standard in most quantum chemistry program systems. However, the basic philosophy of MOLCAS is to be able to treat, at the same level of accuracy also, highly degenerate states, such as those occurring in excited states, at the transition state in some chemical reactions, in biradicaloid systems, in heavy atom systems, etc. This is a more difficult problem since the single determinental approach will not work well in such cases. The key feature of MOLCAS is the multiconfigurational approach. MOLCAS contains codes for general and effective multiconfigurational SCF calculations at the Complete Active Space (CASSCF) level, but also employing more restricted MCSCF wave functions (RASSCF). It is also possible, at this level of theory, to optimize geometries for equilibrium and transition states using gradient techniques and to compute frequencies.