Augmented correlation consistent basis sets

Using the same kind of augmented correlation-consistent basis sets employed in the above coupled cluster methods, but working with the complete active subspace (CASSCF and CASSCF( +1, +2)) approximations to the Cl expansion, Lawson and Harrison101 have investigated the variation with interatomic distance and spatial distribution of the quadrupole moments of P2, S2 and CI2. The a and it contributions to the quadrupole are resolved and the poor results obtained at the SCF level are attributed to the inadequate representation of the it system in the SCF approximation for P2 and S2. 

Del Bene JE (1993) Proton affinities of ammonia, water, and hydrogen fluoride and their anions a quest for the basis-set limit using the Dunning augmented correlation-consistent basis sets. J Phys Chem 97 107-110... 

The interaction electric properties of the HCN-HF and HNC-HF complexes was reported by Li et al Their final-field calculations were performed with large augmented correlation-consistent basis sets. At the CCSD(T) level of theory a large d-aug-cc-pVTZ basis set yields the following values for the dipole moment and (hyper)polarizability invariants of HCN-HF (HNC-HF values are given in parentheses, in italics) p = 2.2918 (2.5335) eao, a = 23.186(24.353) and Aa= 14.393 (74.575) e Eh", =-6.03 (7.60) y = 2553.4 (3049.3) e ao Et l Notice the... 

Table 17 Dissociation Energy, Equilibrium Intemuclear Separation, and Harmonic Frequency from CCSD(T) Calculations on Ne2 with the Singly, Doubly, and Triply Augmented Correlation Consistent Basis Sets. Experimental Values from Ref. 73...

In this subsection we will focus on two reactions F( P) -f H2, a very exoergic reaction, and 0( P) -f H2, a nearly thermoneutral reaction. The calculated reaction energy defects and classical barrier heights for these two reactions from R-UCCSD(T) calculations with the standard and augmented correlation consistent basis sets are summarized in Table 23. 

In Table 25 the PAgS of hydrogen fluoride, carbon monoxide, and ammonia calculated with the CCSD(T) method are listed. Both the standard and augmented correlation consistent basis sets have been considered. The corresponding basis set convergence errors are plotted in Figure 12. A number of conclusion can be drawn from the data in Table 25 and Figure 12. The first is that the accuracy of the CCSD(T) method is at least comparable to the measured values of the electronic proton affinities - in all cases the calculated proton... 

Table 25 Proton Affinities (in kcal mol ) of HF, CO, and NH3 from CCSD(T) Calculations with the Standard and Augmented Correlation Consistent Basis Sets. The HF and CO Results are from Ref. 76, while the NH3 Results are from Ref. 77. Experimental Values were Derived from Refs. 78-80...

The augmented correlation consistent basis sets, when combined with coupled cluster or multireference configuration interaction methods, have been found to provide an accurate description of atomic electron affinities. The calculated EAs of boron and fluorine are summarized in Table 26 and the basis set convergence errors are plotted in Figure 13. The values of the EAs of these two atoms, 6.39 0.23 kcal mol (B) and 78.82 kcal mol" (F), respectively (corrected for spin-orbit effects), bracket the EAs of the other first-row atoms. 

The correlation-consistent basis sets described in Section 8.3.3 have been designed for one particular purpose the accurate calculation of valence-correlated wave functions of ground-state neutral systems. The cc-pVXZ basis sets therefore do not have the flexibility required either for the investigation of core correlation discussed in Section 8.3.1 or for the study of anions and excited states with diffuse electron distributions. For such applications, additional AOs must be introduced. In the present subsection, we shall first discuss the correlation-consistent polarized core—valence sets cc-pCVXZ [24], where the standard cc-pVXZ sets have been extended for additional flexibility in the core region, and next the augmented correlation-consistent basis sets aug-cc-pVXZ and aug-cc-pCVXZ [25], where diffuse functions have been added so as to improve the flexibility in the outer valence region. 

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