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Title: An embedded many-body expansion for molecular crystals
Author: Bygrave , Peter John
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2012
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Reliable prediction of molecular crystal energetics is a vital goal for computational chemistry. Currently researchers mostly rely on DFT for empirical potentials for such applications, But these methods are not systematically improvable and have a reliance on fitting to previous results. Therefore there is a need to develop new methods to perform highly accurate studies of molecular crystals using a monomer-based many-body expansion truncated at the two-body level, we present calculated energetics and optimised structures for periodic solids using high-level quantum mechanical treatments beyond density functional theory. The one- and two-body interactions are calculated using molecular quantum mechanical methods such as MP2 and we demonstrate the straightforward extension to explicitly correlated and coupled cluster theory. The higher order terms in the many-body expansion are included using a new embedding model for the crystalline environment., which takes into account electrostatics and exchange-repulsion. The environment is described using atom-centred Gaussian functions and point charges. The electrostatic potential is derived from these functions, and the exchange-repulsion contributions is derived from a density overlap scheme. The environment density is calculated self-consistently using Hartree-Fock theory. This approach is demonstrated by studying crystalline carbon dioxide, ice Xl and hydrogen fluoride. For carbon dioxide the results approach very closely those from full periodic MP2 calculations. By optimising all structural parameters in these systems we obtain lattice parameters to within 1% of experiment, as well as accurate bond lengths. For example, the calculated average 0- H bond in ice XIh is just 0,002. A from that measured experimentally. We have also studied the relative stability of two phases of ice Xl at the CCSD(T)F12 level. Ice XIh is found to be just 0.1 m Eh per formula unit more stable than the recently proposed ice Xlc phase. For hydrogen fluoride we find. that the contrasting literature reports of either the non-polar or polar phases being most stable may be a result of the two phases having similar cohesive energies and structures; optimised DF-MP2-F12/AVTZ lattice parameters for the two phases agree to within 0.05 A, and the CCSD(T)FI2/AVTZ cohesive energies indicate that the non polar phase is only O. l mE., more stable than the polar phase. Finally, the generality of the embedding method is shown by calculating the cohesive energy of lithium hydride using an embedded hierarchical approach, and we obtain excellent agreement with theoretical values in the literature and experiment.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available