CCSD methods

Clearly, the HF method, independent of basis, systematically underestimates the bond lengdis over a broad percentage range. The CISD method is neither systematic nor narrowly distributed in its errors, but the MP2 and MP4 (but not MP3) methods are reasonably accurate and have narrow error distributions if valence TZ or QZ bases are used. The CCSD(T), but not the CCSD, method can be quite reliable if valence TZ or QZ bases are used. 

Table 3 Second Order Properties Calculated by EOM-CCSD Methods...

In the SOPPA(CCSD) method [36] the Mqller-Plesset correlation coefficients and are replaced in all SOPPA matrix elements by the corresponding coupled cluster singles and doubles amplitudes and r , whereas in the earlier CCSDPPA method [52,53] only some of the Mqller-Plesset correlation coefficients were replaced. Although SOPPA(CCSD) is based on a CCSD wavefunction, it is still only correct through second order and not the linear... 

Harmonic and cubic force fields of Sis were calculated using coupled-cluster (CC) theory (25) and a correlation-consistent basis set. Specifically, the CC singles and doubles (CCSD) method (24) was used in conjunction with the cc-pVTZ basis set (25) developed by Dunning and co-workers. The force... 

A different way to approximate SS-MRCC is the so-called externally corrected CCSD (ec-CCSD) [13-17], The ec-CCSD method is based on the Coupled Cluster Approach (CCA). In CCA the exact wavefunction is written in an exponential form,... 

Comparing (SC) CAS-SDCI and Externally Corrected CCSD Methods... 

For a detailed comparison of (SC)2CAS-SDCI and ec-CCSD methods, equations corresponding to single and double excitations should be written explicitly for both approaches. For the former one, we can substitute eq. (10) in (4) and, using operator notation, we have... 

The present paper is devoted to display the performance of both procedures in situations of streched bonds. These areas of the molecular energy hypersurface are typically placed outside of the domain of good performance of the standard CCSD method [19-23]. In particular, we consider the dissociation of HF, H2O and N2 molecules (i.e., a single bond, two single bonds and a triple bond). 

We have studied the performance of the so-called (SC)2CAS-SDCI and ec-CCSD methods versus standard CAS-SDCI method on one hand, and CCSD on the other hand, in quasidegenerate situations that arise when exploring dissociation channels of simple molecules. In particular, we dissociate a single bond, two single bonds simultaneously and a triple bond. 

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