Direct generation of local orbitals for multireference treatment and subsequent uses for the calculation of the correlation energy

Abstract

We present a method that uses the one-particle density matrix to generate directly localized orbitals dedicated to multireference wave functions. On one hand, it is shown that the definition of local orbitals making possible physically justified truncations of the CAS ͑complete active space͒ is particularly adequate for the treatment of multireference problems. On the other hand, as it will be shown in the case of bond breaking, the control of the spatial location of the active orbitals may permit description of the desired physics with a smaller number of active orbitals than when starting from canonical molecular orbitals. The subsequent calculation of the dynamical correlation energy can be achieved with a lower computational effort either due to this reduction of the active space, or by truncation of the CAS to a shorter set of references. The ground-and excited-state energies are very close to the current complete active space self-consistent field ones and several examples of multireference singles and doubles calculations illustrate the interest of the procedure.

Multi-scale multireference configuration interaction calculations for large systems using localized orbitals: Partition in zones

“…25. It differs from the various methods that can be found in the literature, for example, those by Boys, 26 or Pipek and Mezey.…”

Short range DFT combined with long-range local RPA within a range-separated hybrid DFT framework

“…The localization method [34] we use in this study constructs in a first step local guess orbitals from linear combinations of atom-like, step-wise (core, valence, ...) orthogonalized orbitals from a minimal set of basis functions through the chemical intuition of the bonding in a molecule (σ or π bonding/antibonding orbitals, lone pairs). Occupied orbitals from this construction should represent already closely the electronic SCF density of the molecule, hence the use of an Atomic Natural Orbital (ANO [44,45]) basis set when possible.…”

“…First the RSH equations are solved in canonical orbitals for the monomers in the monomer basis, and for the monomers in the dimer basis. Starting from the first set, the orbitals are localized through a projection procedure, described in reference [34], and recalled in the appendix. This method constructs a minimal set of molecular occupied and virtual orbitals, and adds the remaining virtual space as projected, orthogonalized atomic orbitals.…”

“…The explicit construction of starting orbitals relies on the code of the university of Toulouse [34].…”

Fragment-Localized Kohn−Sham Orbitals via a Singles Configuration-Interaction Procedure and Application to Local Properties and Intermolecular Energy Decomposition Analysis

“…Such results are important as EDA are essential to the development and the calibration of advanced force fields. 11,14 For constructing intrinsically localized DFT molecular orbitals, we take up an old idea, 15 further developed and published by Daudey 16 and still cited recently, 2,17,18 without the need to have delocalized canonical orbitals. The central point is that in Hartree-Fock theory a CI of singly excited Slater determinants lowers the total energy toward a set of Hartree-Fock orbitals, reaching convergence when satisfying Brillouin's theorem: interactions between occupied and virtual orbitals via the Fock matrix vanish.…”