Ramping it up : mixed rampgaussian basis sets
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For decades, allGaussian basis sets have dominated molecular quantum chemistry, despite some of their less desirable properties. In particular, gaussian basis functions cannot describe the nuclearelectron cusp and consequently large numbers of high exponents gaussians are present in core basis functions to try to describe the inner core electron distribution adequately. This is despite the fact that core electron distribution changes little in response to changing molecular environments and...[Show more]
dc.contributor.author  McKemmish, Laura Kelly  

dc.date.accessioned  20181122T00:07:42Z  
dc.date.available  20181122T00:07:42Z  
dc.date.copyright  2015  
dc.date.created  2015  
dc.identifier.other  b3732707  
dc.identifier.uri  http://hdl.handle.net/1885/151264  
dc.description.abstract  For decades, allGaussian basis sets have dominated molecular quantum chemistry, despite some of their less desirable properties. In particular, gaussian basis functions cannot describe the nuclearelectron cusp and consequently large numbers of high exponents gaussians are present in core basis functions to try to describe the inner core electron distribution adequately. This is despite the fact that core electron distribution changes little in response to changing molecular environments and is not an important contribution to chemical energetics that chemists are generally interested in (reaction energies, ionisation energies, isomerisation energies and so on). The principal reason for the dominance of gaussian basis sets is that accurate evaluation of twoelectron integral evaluation in these basis sets is far quicker than for other competing types of basis sets. This thesis seeks to change this. We reintroduce a ramp basis function, which has a nonzero nuclearelectron cusp and can therefore describe the inner core accurately and efficiently. This ramp function has compact support, which greatly reduces the difficulty of twoelectron integral evaluation, but means it cannot describe valence electron distributions. Thus, gaussian basis functions are added to describe valence electrons to form a mixed rampgaussian basis set. To simplify the development of this new class of basis set, we modify only the core basis function in 631G (631+G) to produce R31G (R31+G). These novel rampified basis sets have very similar chemistry to their parent basis sets in atoms and molecules in HartreeFock (HF), density functional theory (DFT) and MollerPlesset 2 Theory (MP2) calculations. The mixed rampgaussian basis sets are vastly better than their parent basis sets at predicting electron density at the nucleus and in fact gives better performance than ccpVQZ. Faster HF, DFT and MP2 calculations in large molecules would increase the speed of the most common types of computational chemistry calculations in the world. This thesis provides strong evidence that mixed rampGaussian basis sets are a viable way to get these faster calculations without compromising chemical accuracy. A preliminary integral evaluation program, RampItUp, was developed in this thesis that calculates all one and twoelectron rampcontaining integrals for a mixed rampGaussian basis set with Sramps, sand pgaussians. With the main caveat that twoelectron screening is not performed in either basis set, the Fock build time for R31+G is faster than in 631+G by about 10% for large linear molecules, e.g. fatty acids, with more than 20 heavy atoms. This demonstrates that fast integral evaluation is possible. This thesis provides strong justification for the inclusion of rampgaussian basis sets into mainstream quantum chemistry packages to allow the full benefits of these new type of basis sets to be experienced by the field as a whole.  
dc.format.extent  xxviii, 180 leaves.  
dc.language.iso  en_AU  
dc.rights  Author retains copyright  
dc.title  Ramping it up : mixed rampgaussian basis sets  
dc.type  Thesis (PhD)  
local.description.notes  Thesis (Ph.D.)Australian National University  
local.type.status  Accepted Version  
local.contributor.affiliation  Australian National University. Research School of Chemistry  
local.identifier.doi  10.25911/5d5153d25e971  
dc.date.updated  20181121T08:18:10Z  
dcterms.accessRights  Open Access  
local.mintdoi  mint  
Collections  Open Access Theses 
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