Equilibrium geometries basis sets

The next step in iin proving a basis set could be to go to triple zeta, quadruple zeta, etc. Ifone goes in this direction rather than adding functions of higher angular quantum number, the basis set would not be well balanced. With a large number of s and p functions only, one finds, for example, that the equilibrium geometry of am monia actually becomes planar. The next step beyond double z.ela n sit ally in voices addin g polarization fn n ciion s, i.e.. addin g d-... 

Transition structures are more dihicult to describe than equilibrium geometries. As such, lower levels of theory such as semiempirical methods, DFT using a local density approximation (LDA), and ah initio methods with small basis sets do not generally describe transition structures as accurately as they describe equilibrium geometries. There are, of course, exceptions to this, but they must be identihed on a case-by-case basis. As a general rule of thumb, methods that are empirically dehned, such as semiempirical methods or the G1 and G2 methods, describe transition structures more poorly than completely ah initio methods do. 

Optimize the geometry of this system at the Hartree-Fock level, using the STO-3G minimal basis set and the 6-31G(d) basis set (augmented as appropriate). Run a frequency calculation following each optimization in order to confirm that you have found an equilibrium structure. 

Beginning with the final optimized structure from step 1, obtain the fii equilibrium geometry using the fuU MP2 method—requested with t MP2(Full keyword in the route section—which includes inner sh electrons. The 6-31G(d) basis set is again used. This geometry is used 1 all subsequent calculations. 

H.Koch, R. Kobayashi, A. Sanchez de Meras and P. Jprgensen, J. Chem. Phys., 100, 4393 (1994) The basis set and the geometry used in Ref 16 and 17 are different from the present work. We used commonly used basis set and equilibrium geometry for calibrating more realistic estimate of the effects. 

All three states were described by a single set of SCF molecular orbitals based on the occupied canonical orbitals of the X Z- state and a transformation of the canonical virtual space known as "K-orbitals" [10] which, among other properties, approximate the set of natural orbitals. Transition moments within orthogonal basis functions are easier to derive. For the X state the composition of the reference space was obtained by performing two Hartree-Fock single and double excitations (HFSD-CI) calculations at two typical intemuclear distances, i.e. R. (equilibrium geometry) and about 3Re,and adding to the HF... 

The dependence of the DFT results on the basis set used to expand the Kohn-Sham orbitals is illustrated in Table 4.3, which collects equilibrium geometry properties of water dimer obtained with the same exchange-correlation functional (B88/P86) but with different basis sets. 

The intrinsic atom-in-molecule polarizabilities were tested by us [99] for their performance in the calculation of the dispersion energy for a set of Van der Waals complexes, at their equilibrium geometry using a DFT- B3LYP computational ansatz combined with an aug-cc-pVTZ basis set for the calculation of the (Mf) values using Van Alsenoy s STOCK program, also used to partition the polarizabilities [100]. 

In order to see whether this kind of intermolecular energy partitioning also works well in ion-molecule complexes, and to study basis set effects on the results, we performed a number of calculations on Li+... OH2 91h The numerical values obtained are shown in Table 3. To facilitate comparison the geometry of the water molecule was frozen at the experimental values and the intermolecular distance was fixed to i ou, = 3.4 a. 1. u. 1.8 A, which is very near to the equilibrium value. 

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