Multireference space without first solving the configuration interaction problem

• Published in: Journal of computational chemistry Vol. 35; no. 4; pp. 313 - 323
• Main Authors: Glushkov, Vitaly N., Assfeld, Xavier
• Format: Journal Article
• Language: English
• Published: United States Blackwell Publishing Ltd 05.02.2014; Wiley Subscription Services, Inc
• Subjects: correlation energy; excited states; Mathematical functions; Mathematical problems; Molecular chemistry; Molecules; multireference perturbation theory; orthogonality constraints; Quantum physics; Vector space; correlation energy; orthogonality constraints; multireference perturbation theory; excited states
• ISSN: 0192-8651; 1096-987X; 1096-987X
• DOI: 10.1002/jcc.23502

We further develop an idea to generate a compact multireference space without first solving the configuration interaction problem previously proposed for the ground state (GS) (Glushkov, Chem. Phys. Lett. 1995, 244, 1). In the present contribution, our attention is focused on low‐lying excited states (ESs) with the same symmetry as the GS which can be adequately described in terms of an high‐spin open‐shell formalism. Two references Møller–Plesset (MP) like perturbation theory for ESs is developed. It is based on: (1) a main reference configuration constructed from the parent molecular orbitals adjusted to a given ES and (2) secondary double excitation configuration built on the GS like orbitals determined by the Hartree–Fock equations subject to some orthogonality constraints. It is shown how to modify the MP zeroth‐order Hamiltonian so that the reference configurations and corresponding excitations are eigenfunctions of it and are compatible with orthogonality conditions for the GS and ES. Intruder states appearance is also discussed. The proposed scheme is applied to the GS, ES, and excitation energies of small molecules to illustrate and calibrate our calculations. © 2013 Wiley Periodicals, Inc. A method is presented for generating a compact multireference space for the ground and excited states without first solving the configuration interaction problem. The proposed scheme is applied to small molecules to illustrate and calibrate calculations. The expressions for the second‐order corrections of many‐body perturbation theory based on this multireference space keep the computational advantages of the genuine Møller–Plesset scheme.