RCCSD method

One should be aware that, similar to the case of RMP2, there is no unique RCCSD method.Also, there are slightly different ways for the noniterative evaluation of the T3 contributions in open-shell systems. Therefore, CCSD(T) energies obtained from different programs may differ so the program used should be specified when the results of CCSD(T) (or, for that matter, any) calculations are reported. 

Paldus, elsewhere in this book, discusses that there is as yet no generally applicable, open-shell, size-extensive, coupled cluster method, and the same holds for open-shell S APT methods. Therefore, for the computation of potentials of open-shell van der Waals molecules one has the choice between CASSCF followed by a Davidson-corrected MRCl calculation of the interaction energy, or the single reference, high spin, method RCCSD(T). When the ground state of the open-shell monomer is indeed a high spin state, then RCCSD(T) is the method of choice. With regard to the latter method we recall that a major difficulty in open-shell systems is the adaptation of the wave function to the total spin operator S for the CCSD method a partial spin adaptation was published by Knowles et al. [219,220] who refer to their method as partially spin restricted . When non-iterative triple corrections [221] are included, the spin restricted CCSD(T) method, RCCSD(T), is obtained. 

We used the aug-cc-pv(5 -I- d)z basis for P [20] and the aug-cc-pv5z basis set for F and Cl [21], and in all calculations, we restricted the angular momentum on the basis of spdf. This was primarily because the locally written codes that performed the subsequent Hirshfeld analysis are limited to basis functions with these symmetries. We characterized the molecules using the SCF, CASSCF, the internally contracted mrci and the RCCSD(t) methods as implemented in MOLPRO [22], and in all cases, we only... 

In Table 7, optimized C2V bond angles and stabilization energies with respect to the D3h saddle point are summarized for the nine cations. In both the LC-B97D and RCCSD(T) methods, two C2v geometries were optimized for each system, with a potential maximum at D3h symmetry. Such results are indicative of strong pseudo Jahn-... 

Table 7 Comparison of optimized angles and energy differences AE = E(C2v) — E(D3h) obtained by LC-B97D, RCCSD(T) and B3PW91 methods...

By virtue of the infinite-order feature of the CC method, its ability to rectify shortcomings in Pq is much enhanced, compared to MPn calculations. This proves particularly advantageous in radicals and radical ions, in cases where neither UHF nor ROHF reference wavefunctions are satisfactory starting points for MPn calculations. It has been demonstrated that CCSD is quite efficient at reducing UHF spin contamination to acceptable levels, so the results of UHF- and ROHF-based CCSD calculations are often fairly similar. This is shown in Table 1, which also reveals that RCCSD(T) gives results that are almost identical to CAS-CISD, after the Davidson correction has been applied to the latter. 

DFT offers, through TD-DFT, advantages on the ease of calculation of the transition energies to the excited states compared with restricted methods such as RMP2 or RCCSD(T). Since the smdy of halogens in condensed phase solutions is aimed to understand the solvatochromic effect, here we decided to perform test calculations with the ubiquitous B3LYP functional and a larger basis set than for MP2 calculations, AVTZ instead of AVDZ. An exhaustive exploration of different levels within the DFT framework is beyond the scope of this work. 

Table 12 Calculated and Experimental Values for and of CH, HF, N2, and CO. Calculations for HF, N2, and CO Used the CCSD(T) Method for CH the RCCSD(T) Method was Used. The Complete Basis Set (CBS) Limits were Obtained from an Analysis of the Behavior of AQbs( )- Experimental Values from Ref. 72...

The results of large basis set ab initio electronic structure calculations using the RCCSD(T) method are reported for the bond lengths, bond energies, excitation energies, vibrational frequencies, dipole moments and charge distributions for the titled molecules and where possible compared with experiment and previous calculations The striking differences between the Ca and Zn compounds are discussed in terms of their relative ionic character. 

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