29 research outputs found
Prediction of many-electron wavefunctions using atomic potentials: refinements and extensions to transition metals and large systems
For a given many-electron molecule, it is possible to define a corresponding
one-electron Schr\"odinger equation, using potentials derived from simple
atomic densities, whose solution predicts fairly accurate molecular orbitals
for single- and multi-determinant wavefunctions for the molecule. The energy is
not predicted and must be evaluated by calculating Coulomb and exchange
interactions over the predicted orbitals. Transferable potentials for first-row
atoms and transition metal oxides that can be used without modification in
different molecules are reported. For improved accuracy, molecular
wavefunctions can be refined by slightly scaling nuclear charges and by
introducing potentials optimized for functional groups. For a test set of 20
molecules representing different bonding environments, the transferable
potentials with scaling give wavefunctions with energies that deviate from
exact self-consistent field or configuration interaction energies by less than
0.05 eV and 0.02 eV per bond or valence electron pair, respectively.
Applications to the ground and excited states of a Ti18O36 nanoparticle and
chlorophyll-s are reported.Comment: 20 pages. arXiv admin note: text overlap with arXiv:1702.0685
Prediction of many-electron wavefunctions using atomic potentials: extended basis sets and molecular dissociation
A one-electron Schroedinger equation based on special one-electron potentials
for atoms is shown to exist that produces orbitals for an arbitrary molecule
that are sufficiently accurate to be used without modification to construct
single- and multi-determinant wavefunctions. The exact Hamiltonian is used to
calculate the energy variationally and to generate configuration interaction
expansions. Earlier work on equilibrium geometries is extended to larger basis
sets and molecular dissociation. For a test set of molecules representing
different bonding environments, a single set of invariant atomic potentials
gives wavefunctions with energies that deviate from configuration interaction
energies based on SCF orbitals by less than 0.04 eV per bond or valence
electron pair. On a single diagonalization of the Fock matrix, the
corresponding errors are reduced 0.01 eV. Atomization energies are also in good
agreement with CI values based on canonical SCF orbitals. Configuration
interaction applications to single bond dissociations of water and glycine, and
multiple bond dissociations of ethylene and oxygen produce dissociation energy
curves in close agreement with CI calculations based on canonical SCF orbitals
for the entire range of internuclear distances
Estimates of electron correlation based on density expansions
Methods for estimating the correlation energy of molecules and other
electronic systems are discussed based on the assumption that the correlation
energy can be partitioned between atomic regions. In one method, the electron
density is expanded in terms of atomic contributions using rigorous electron
repulsion bounds, and, in a second method, correlation contributions are
associated with basis function pairs. The methods do not consider the detailed
nature of localized excitations, but instead define a correlation energy per
electron factor that that is unique to a specific atom. The correlation factors
are basis function dependent and are determined by from configuration
interaction calculations on diatomic and hydride molecules. The correlation
energy estimates are compared with the results of high-level configuration
interaction calculations for a test set of twenty-seven molecules representing
a wide range of bonding environments (average error of 2.6%). An extension
based on truncated CI calculations in which d- and hydrogen p-type functions
are eliminated from the virtual space combined with estimates of dynamical
correlation contributions using atomic correlation factors is discussed and
applied to the dissociation of several molecules.Comment: 22 pages, 4 figure
Final Report Theoretical Studies of Surface Reactions on Metals and Electronic Materials
This proposal describes the proposed renewal of a theoretical research program on the structure and reactivity of molecules adsorbed on transition metal surfaces. A new direction of the work extends investigations to interfaces between solid surfaces, adsorbates and aqueous solutions and includes fundamental work on photoinduced electron transport into chemisorbed species and into solution. The goal is to discover practical ways to reduce water to hydrogen and oxygen using radiation comparable to that available in the solar spectrum. The work relates to two broad subject areas: photocatalytic processes and production of hydrogen from water. The objective is to obtain high quality solutions of the electronic structure of adsorbate-metal-surface-solution systems so as to allow activation barriers to be calculated and reaction mechanisms to be determined. An ab initio embedding formalism provides a route to the required accuracy. New theoretical methods developed during the previous grant period will be implemented in order to solve the large systems involved in this work. Included is the formulation of a correlation operator that is used to treat localized electron distributions such as ionic or regionally localized distributions. The correlation operator which is expressed as a two-particle projector is used in conjunction with configuration interaction
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Final Report Theoretical Studies of Surface Reactions on Metals and Electronic Materials
This proposal describes the proposed renewal of a theoretical research program on the structure and reactivity of molecules adsorbed on transition metal surfaces. A new direction of the work extends investigations to interfaces between solid surfaces, adsorbates and aqueous solutions and includes fundamental work on photoinduced electron transport into chemisorbed species and into solution. The goal is to discover practical ways to reduce water to hydrogen and oxygen using radiation comparable to that available in the solar spectrum. The work relates to two broad subject areas: photocatalytic processes and production of hydrogen from water. The objective is to obtain high quality solutions of the electronic structure of adsorbate-metal-surface-solution systems so as to allow activation barriers to be calculated and reaction mechanisms to be determined. An ab initio embedding formalism provides a route to the required accuracy. New theoretical methods developed during the previous grant period will be implemented in order to solve the large systems involved in this work. Included is the formulation of a correlation operator that is used to treat localized electron distributions such as ionic or regionally localized distributions. The correlation operator which is expressed as a two-particle projector is used in conjunction with configuration interaction