title: Three- and four-body nonadditivities in nucleic acid tetramers: a CCSD(T) study.
abstract: Three- and four-body nonadditivities in the uracil tetramer (in DNA-like geometry) and the GC step (in crystal geometry) were investigated at various levels of the wave-function theory: HF, MP2, MP3, L-CCD, CCSD and CCSD(T). All of the calculations were performed using the 6-31G**(0.25,0.15) basis set, whereas the HF, MP2 and the MP3 nonadditivities were, for the sake of comparison, also determined with the much larger aug-cc-pVDZ basis set. The HF and MP2 levels do not provide reliable values for many-body terms, making it necessary to go beyond the MP2 level. The benchmark CCSD(T) three- and four-body nonadditivities are reasonably well reproduced at the MP3 level, and almost quantitative agreement is obtained (fortuitously) either on the L-CCD level or as an average of the MP3 and the CCSD results. Reliable values of many-body terms (especially their higher-order correlation contributions) are obtained already when the rather small 6-31G**(0.25,0.15) basis set is used. The four-body term is much smaller when compared to the three-body terms, but it is definitely not negligible, e.g. in the case of the GC step it represents about 16% of all of the three- and four-body terms. While investigating the geometry dependence of many-body terms for the GG step at the MP3/6-31G**(0.25,0.15) level, we found that it is necessary to include at least three-body terms in the determination of optimal geometry parameters.
authors.affiliation: Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, v. v. i. and Center for Biomolecules and Complex Molecular Systems, Flemingovo nám. 2, 166 10 Praha 6, Czech Republic.
authors.forename: M
authors.initials: M
authors.lastname: Pitonák
authors.forename: P
authors.initials: P
authors.lastname: Neogrády
authors.forename: P
authors.initials: P
authors.lastname: Hobza
mesh_terms: D020029:Base Pairing; D001483:Base Sequence; D003596:Cytosine / Q000737:chemistry; D005240:Feasibility Studies; D006147:Guanine / Q000737:chemistry; D006860:Hydrogen Bonding; D008958:Models, Molecular; D009696:Nucleic Acids / Q000737:chemistry* / Q000235:genetics; D011108:Polymers / Q000737:chemistry*; D011789:Quantum Theory; D013816:Thermodynamics; D014498:Uracil / Q000737:chemistry
chemical_list: D009696:Nucleic Acids; D011108:Polymers; D014498:Uracil; D006147:Guanine; D003596:Cytosine