Dynamical electron-proton correlation in the nuclear-electronic orbital framework
Open Access
- Author:
- Swalina, Chester W
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 21, 2006
- Committee Members:
- Sharon Hammes Schiffer, Committee Chair/Co-Chair
Barbara Jane Garrison, Committee Member
James David Kubicki, Committee Member
Karl Todd Mueller, Committee Member - Keywords:
- dynamical correlation
electron
proton - Abstract:
- Methods for including dynamical electron-proton correlation within the nuclear-electronic orbital (NEO) framework are explored. In the NEO approach, nuclear quantum effects are incorporated directly within electronic structure calculations by treating specified nuclei quantum mechanically on the same level as the electrons. Mixed nuclear electronic wavefunctions are calculated by solving the time independent nuclear-electronic Schrodinger equation using molecular orbital techniques. A formulation of nuclear-electronic orbital many-body perturbation theory (NEO-MP2) for recovering electron-proton correlation is presented. The molecular structures for a series of bihalides and the hydrogen fluoride (HF) dimer are calculated at the NEO-MP2 level and compared to vibrationally averaged structures obtained using conventional Born-Oppenheimer based approaches and to experimental values. The bihalide X-X distances are similar for all methods. For the HF dimer, the NEO-MP2 F-F distance is in excellent agreement with the distance obtained experimentally for a model which removes the large amplitude bending motions. The NEO-MP2 approach does not include these large amplitude bending motions because it does not recover enough dynamical electron-proton orrelation. A method which incorporates explicit electron-proton correlation directly into the nuclear-electronic orbital self consistent-field framework is presented. This nuclear-electronic orbital explicitly correlated Hartree-Fock (NEO-XCHF) scheme is formulated using Gaussian basis functions for the electrons and the quantum nuclei in conjunction with Gaussian type geminal functions. The description of the nuclear wavefunction is significantly improved by the inclusion of explicit electron-proton correlation. The NEO-XCHF method leads to hydrogen vibrational stretch frequencies that are in excellent agreement with those calculated from grid-based methods. Overall, these findings demonstrate that dynamical electron proton correlation significantly impacts the qualitative characteristics of nuclear wavefunctions. Moreover, in the NEO framework, dynamical electron-proton correlation is even more important than dynamical electron-electron correlation. Methods based on orbital expansions for recovering dynamical electron proton correlation suffer from slow convergence reminiscent of that found for precise treatments of electron-electron correlation. Therefore, an explicit treatment of electron-proton correlation is required.