There are multiple types of functionals and basis sets that can be used for different calculations in Gaussian such as optimizations, scans, and excited state energy calculations. A basis set is a set of basis functions. Each basis set is a different size and generally, the bigger the basis set size, the more accurate the results will be. The names of the basis sets accessible through Gaussian are 6-31G (which can include +,++, and different orbitals), STO-3G, 3-21G, 6-311G, cc-pVDZ, cc-pVTZ, cc-pVQZ, LanL2DZ, LanL2MB, SDD, DGDZVP, DGDZVP2, DGTZVP, GEN, and GENECP. However, really there are many more options available which are discussed more thoroughly on the following website (http:\/\/www.gaussian.com\/g_tech\/g_ur\/m_basis_sets.htm<\/a>). It is also possible to create your own basis set using Gaussian, but this can be time-consuming and complicated. In relation to 6-31G, the increasing size of the basis set in terms of +, ++, aug- (which are augmented basis sets) \u00a0and p,d,f orbitals or *,** (polarization functions), the more that are included, the more accurate results these should be as well. Each basis set contains a different number of Cartesian (etc) basis functions, which can be found in the output file (ctrl-f “basis function”). The larger the number of basis functions corresponds to a longer calculation time.<\/p>\n I performed an optimization calculation on a new conformation of tryptophan and then ran excited state calculations using 16 combinations of functionals\u00a0(b3lyp, cam-b3lyp, pbepbe, and wb97xd) and basis sets\u00a0(6-31G, 6-31+G, 6-31+G(d,p), and cc-pVDZ). Since 6-31G is the smallest basis set here, it took the shortest time to complete calculations in all of the functionals. Also, within functionals, cc-pVDZ is similar in time to 6-31G. Below is a table showing the times and number of basis functions for each basis set that was used in calculating excited state energies of an optimized configuration of tryptophan.<\/p>\n A) Basis set:\u00a06-31G<\/p>\n Cartesian Basis functions: 159<\/p>\n
| Functional<\/td>\n | b3lyp<\/td>\n | cam-b3lyp<\/td>\n | PBEPBE<\/td>\n | wB97XD<\/td>\n<\/tr>\n | |||||||||||||||||||||||||||||||||||||||||||||
| Job CPU Time \/ Minutes<\/td>\n | 7.733<\/td>\n | 9.717<\/td>\n | 7.45<\/td>\n | 10.1<\/td>\n<\/tr>\n | |||||||||||||||||||||||||||||||||||||||||||||
| <\/td>\n | <\/td>\n | <\/td>\n | <\/td>\n | <\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n <\/p>\n B) Basis set:\u00a06-31+G<\/p>\n Cartesian Basis functions: 219<\/p>\n
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