Calculated using coupled-cluster potential energy surface

Recent years have witnessed a considerable activity towards extending the standard single-reference coupled-cluster (CC) methods (1-9) to potential energy surfaces (PESs) involving bond breaking without invoking a multireference description (see, e.g., refs 9-31). Undoubtedly, it would be very useful if we could routinely calculate large portions of molecular PESs with the ease-... 

The results of Harrison and Bartlett illustrate that the problem of large correlated BSSE pertains to methods other than Mpller-Plesset, e.g., coupled cluster approach. Baum and Finney carried out their correlated calculations of the water dimer using a Cl procedure. They advocated using basis sets of high enough quality that the BSSE is reduced so as to minimize distortion of the potential energy surface, but noted the difficulty in achieving such low superposition error at correlated levels. 

Two coupling modes are considered for the Pdj CO cluster the first mode (denoted as h) represents vibration of the rigid CO molecule with respect to the transition metal surface. The second mode is either the Pd-Pd vibration wi in the plane of Pd surface atoms (r) or out-of-plane stretch of the surface/sub-surface Pd-Pd bond (z). The total energy surfaces (h,r) and (h,z) are calculated for discrete points and then fitted to a fourth order polynomial. Variational and Quantum Monte Carlo (QMC) methods were subsequently applied to calculate the ground and first excited vibrational states of each two-dimensional potential surfaces. The results of the vibrational frequences (o using both the variational and QMC approach are displayed in Table II. 

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