Evaluating the Efficacy of Small Basis Sets and the Counterpoise Procedure to Reproduce Complete Basis Set Limit Higher-Order Correlation Corrections for Weakly Bound Molecular Clusters

Thorne, Cara M. (2014) Evaluating the Efficacy of Small Basis Sets and the Counterpoise Procedure to Reproduce Complete Basis Set Limit Higher-Order Correlation Corrections for Weakly Bound Molecular Clusters. Undergraduate thesis, under the direction of Gregory S. Tschumper from Chemistry & Biochemistry, The University of Mississippi.

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Abstract

The efficacy of small basis sets and the counterpoise procedure to accurately reproduce higher-order correlation corrections, which are routinely employed to estimate CCSD(T) interaction energies at the complete basis set (CBS) limit using an additive scheme, was examined through the study of four dispersion-dominated dimer systems. The dimers of acetylene (HCCH), cyanogen (NCCN), diphosphorous (P2) and 1,4-diphosphabutadiyne (PCCP) were studied in three different configurations: cross, parallel-displaced and t-shaped at the intermolecular distance corresponding to the minimum point on the CCSD(T) potential energy curve (PEC) for each system using MP2, CCSD and CCSD(T) methods in conjunction with eight basis sets from Dunning’s correlation consistent set of basis sets both with and without diffuse functions. Six standard extrapolation techniques were used to produce CBS limit estimates of the higher-order correlation corrections based on the differences between CCSD(T) and MP2 interaction energies and seven standard extrapolation techniques were used to produce the corresponding differences between CCSD(T) and CCSD interactions energies. Triple-�zeta quality basis sets are required to accurately reproduce the �CCSD(T) MP2 higher-order correlation correction to within 0.1 kcal mol−1 while double-�zeta quality basis sets with diffuse functions can reproduce the �CCSD(T) CCSD higher-order correlation correction to within 0.1 kcal mol−1. Only when the aDZ basis set was used to compute the �CCSD(T) MP2 higher-order correlation correction did the counterpoise procedure improve agreement of the higher-order correlation corrections to the CBS limit compared with non-corrected results. The counterpoise procedure rarely yielded a better description of the �CCSD(T) CCSD higher-order correlation correction relative to the values obtained without the counterpoise procedure.

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