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Closely approximating second-order Mo/ller-Plesset perturbation theory with a local triatomics in molecules model Noniterative local second order Mo/ller-Plesset theory: Convergence with local correlation space Fourth-order M011er-Plesset perturbation theory (MP4) is formulated for localized internal orbitals of closed-shell systems. Unlike previous localized perturbation theories, our formulation is strictly identical with canonical MP4 theory if no further approximations are made. In the local treatment, large savings can be achieved by two techniques: (1) neglecting or treating at a lower (second order) level pair correlation between distant pairs, and (2) restricting the correlation basis to the atomic orbitals in the spatial vicinity of the correlated pair. These techniques have been used in our previous local correlation treatment for variational CI, coupled electron pair (CEPA), and approximate coupled cluster (ACCD) wave functions. The MP4 method is more economical than these techniques because of the absence of iterative cycles. Implementation with single, double, and quadruple substitutions is discussed.914

The water dimer has been studied by accurate ab initio calculations. The main purpose of the calculations was to investigate the magnitude of, and how to eliminate the basis set superposition errors at different levels of theory. At the Hartree–Fock level the superposition errors are insignificant with the largest basis sets, and the counterpoise method works well with all the basis sets used in this study. At the correlated level superposition errors are still significant even for very large basis sets, and the standard counterpoise technique leads to overcorrection. The most important result of the present study is that the local correlation methods gives essentially the correct result for the correlation contribution to the association energy even with modest basis sets. The association energy at the MP4(SDQ) level is predicted to be 4.8 kcal/mol. The correlation contribution to the association energy is 1.2 kcal/mol which can be decomposed into an attractive intermolecular contribution of 1.8 kcal/mol and a repulsive intramolecular contribution of 0.6 kcal/mol. Ionic terms contribute about 30% to the dispersion force at the equilibrium distance. If the effect of triple substitutions is taken into account the association energy is estimated to be around 5.1 kcal/mol.

Computational results are reported which demonstrate the performance of the local correlation method, including convergence behavior and supercomputer timing. Calculations at the local MP4(SDQ) level with the 6-311G** basis have been carried out for molecules as big as octatetraene, demonstrating that, using the local correlation approach, highly accurate calculations are feasible for quite large molecules.

This article describes the capabilities and performance of the latest release (version 4.0) of the Parallel Quantum Solutions (PQS) ab initio program package. The program was first released in 1998 and evolved from the TEXAS program package developed by Pulay and coworkers in the late 1970s. PQS was designed from the start to run on Linux-based clusters (which at the time were just becoming popular) with all major functionality being (a) fully parallel; and (b) capable of carrying out calculations on large-by ab initio standards-molecules, our initial aim being at least 100 atoms and 1000 basis functions with only modest memory requirements. With modern hardware and recent algorithmic developments, full accuracy, high-level calculations (DFT, MP2, CI, and Coupled-Cluster) can be performed on systems with up to several thousand basis functions on small (4-32 node) Linux clusters. We have also developed a graphical user interface with a model builder, job input preparation, parallel job submission, and post-job visualization and display.