Multicomponent quantum chemistry, in which more than one type of particle is described quantum mechanically, is attracting a substantial amount of attention within the quantum chemistry community, and the accurate description of multicomponent systems presents a major challenge. The nuclear-electronic orbital (NEO) approach1 offers a framework for the accurate and efficient description of multicomponent systems. Within the NEO approach, select nuclei, as well as all electrons, are treated quantum mechanically with molecular orbital techniques. The simplest approach within the NEO framework is the Hartree-Fock (NEO-HF) method,1 which is plagued by the lack of electron-electron and electron-proton correlation effects. The next obvious step is to include these correlation effects perturbatively2 or by the configuration interaction method,1 but earlier studies have shown that these approaches fail to incorporate a sufficient degree of electron-proton correlation. After discussing this progression of multicomponent wave function methods, this talk will present an efficient implementation of the coupled cluster singles and doubles method within the NEO framework (NEO-CCSD).3 Because inclusion of single excitations in the NEO-CCSD method is shown to play a crucial role in the level of accuracy, the multicomponent orbital optimized NEO-CCD approach will also be discussed. Applications of these methods to computing proton affinities and proton densities, as well as other molecular properties, will be presented to highlight the capabilities of multicomponent wavefunction methods.

Multicomponent wave function based methods for describing nuclear quantum effects in molecular systems

2019 Fall National Meeting

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