6.18.3 Coupled Cluster Valence Bond (CCVB)

Cases where PP needs improvement include molecules with several strongly correlated electron pairs that are all localized in the same region of space, and therefore involve significant inter-pair, as well as intra-pair correlations. For some systems of this type, Coupled Cluster Valence Bond (CCVB) is an appropriate method. ¹¹¹² Small D. W., Head-Gordon M.
J. Chem. Phys.
(2009), 130, pp. 084103. Link ^{, 1113} Small D. W., Head-Gordon M.
Phys. Chem. Chem. Phys.
(2011), 13, pp. 19285. Link CCVB is designed to qualitatively treat the breaking of covalent bonds. At the most basic theoretical level, as a molecular system dissociates into a collection of open-shell fragments, the energy should approach the sum of the ROHF energies of the fragments. CCVB is able to reproduce this for a wide class of problems, while maintaining proper spin symmetry. Along with this, CCVB’s main strength, come many of the spatial symmetry breaking issues common to the GVB-CC methods.

Like the other methods discussed in this section, the leading contribution to the CCVB wave function is the perfect pairing wave function, which is shown in Eq. (6.55). One important difference is that CCVB uses the PP wave function as a reference in the same way that other GVBMAN methods use a reference determinant.

The PP wave function is a product of simple, strongly orthogonal singlet geminals. Ignoring normalization, two equivalent ways of displaying these geminals are

where on the left and right we have the spatial part (involving $\varphi$ and $\chi$ orbitals) and the spin coupling, respectively. The VB-form orbitals are non-orthogonal within a pair and are generally AO-like. The VB form is used in CCVB and the NO form is used in the other GVBMAN methods. It turns out that occupied UHF orbitals can also be rotated (without affecting the energy) into the VB form (here the spin part would be just $\alpha \beta$), and as such we store the CCVB orbital coefficients in the same way as is done in UHF (even though no one spin is assigned to an orbital in CCVB).

These geminals are uncorrelated in the same way that molecular orbitals are uncorrelated in a HF calculation. Hence, they are able to describe uncoupled, or independent, single-bond-breaking processes, like that found in C${}_2$H${}_6$ $\to$ 2 CH${}_3$, but not coupled multiple-bond-breaking processes, such as the dissociation of N${}_2$. In the latter system the three bonds may be described by three singlet geminals, but this picture must somehow translate into the coupling of two spin-quartet N atoms into an overall singlet, as found at dissociation. To achieve this sort of thing in a GVB context, it is necessary to correlate the geminals. The part of this correlation that is essential to bond breaking is obtained by replacing clusters of singlet geminals with triplet geminals, and re-coupling the triplets to an overall singlet. A triplet geminal is obtained from a singlet by simply modifying the spin component accordingly. We thus obtain the CCVB wave function:

In this expansion, the summations go over the active singlet pairs, and the indices shown in the labellings of the kets correspond to pairs that are being coupled as described just above. We see that this wave function couples clusters composed of even numbers of geminals. In addition, we see that the amplitudes for clusters containing more than 2 geminals are parameterized by the amplitudes for the 2-pair clusters. This approximation is important for computational tractability, but actually is just one in a family of CCVB methods: it is possible to include coupled clusters of odd numbers of pairs, and also to introduce independent parameters for the higher-order amplitudes. At present, only the simplest level is included in Q-Chem.

Older methods which attempt to describe substantially the same electron correlation effects as CCVB are the IP ¹²¹³ Van Voorhis T., Head-Gordon M.
Chem. Phys. Lett.
(2000), 317, pp. 575. Link and RCC ¹²¹⁶ Van Voorhis T., Head-Gordon M.
J. Chem. Phys.
(2001), 115, pp. 7814. Link wave functions. In general CCVB should be used preferentially. It turns out that CCVB relates to the GVB-IP model. In fact, if we were to expand the CCVB wave function relative to a set of determinants, we would see that for each pair of singlet pairs, CCVB contains only one of the two pertinent GVB-IP doubles amplitudes. Hence, for CCVB the various computational requirements and timings are very similar to those for GVB-IP. The main difference between the two models lies in how the doubles amplitudes are used to parameterize the quadruples, sextuples, etc., and this is what allows CCVB to give correct energies at full bond dissociation.

A CCVB calculation is invoked by setting CORRELATION = CCVB. The number of active singlet geminals must be specified by GVB_N_PAIRS. After this, an initial guess is chosen. There are three main options for this, specified by the following keyword

For potential energy surfaces, restarting from a previously computed CCVB solution is recommended. This is invoked by GVB_RESTART = TRUE. Whenever this is used, or any time orbitals are being read directly into CCVB from another calculation, it is important to also set:

• •

  SCF_GUESS = READ

• •

  MP2_RESTART_NO_SCF = TRUE

• •

  SCF_ALGORITHM = DIIS

This bypasses orthogonalization schemes used elsewhere within Q-Chem that are likely to jumble the CCVB guess.

In addition to the parent CCVB method as discussed up until now, we have included two related schemes for energy optimization, whose operation is controlled by the following keyword: