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Molpro is a general purpose quantum chemistry software package with a long development history. It was originally focused on accurate wavefunction calculations for small molecules but now has many additional distinctive capabilities that include, inter alia, local correlation approximations combined with explicit correlation, highly efficient implementations of single-reference correlation methods, robust and efficient multireference methods for large molecules, projection embedding, and anharmonic vibrational spectra. In addition to conventional input-file specification of calculations, Molpro calculations can now be specified and analyzed via a new graphical user interface and through a Python framework.

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An automatic procedure for the generation of potential energy surfaces based on high level ab initio calculations is described. It allows us to determine the vibrational wave functions for molecules of up to ten atoms. Speedups in computer time of about four orders of magnitude in comparison to standard implementations were achieved. Effects due to introduced approximations--within the computation of the potential--on fundamental modes obtained from vibrational self-consistent field and vibrational configuration interaction calculations are discussed. Benchmark calculations are provided for formaldehyde and 1,2,5-oxadiazole (furazan).

The structures and stabilities of small water clusters are studied by local electron correlation methods. It is demonstrated that the local treatment eliminates basis set superposition errors (BSSEs) to a large extent and thus allows BSSE-free geometry optimizations. Results for various basis sets are presented which show that the interaction energies and structural parameters obtained by local second-order Møller−Plesset perturbation theory (LMP2) without counterpoise correction are in close agreement with counterpoise-corrected conventional MP2 results. Furthermore, a partitioning of the LMP2 energies of (H2O) n , n = 2−4, into different excitation classes is reported, which underlines the importance of ionic contributions as well as intramolecular correlation for hydrogen-bonded clusters. The results of this analysis are compared with previous data obtained by symmetry-adapted perturbation theory (SAPT).

Based on the orbital invariant formulation of Mo/ller–Plesset (MP) perturbation theory, analytical energy gradients have been formulated and implemented for local second order MP (LMP2) calculations. The geometry-dependent truncation terms of the LMP2 energy have to be taken into account. This leads to a set of coupled-perturbed localization (CPL) equations which must be solved together with the coupled-perturbed Hartree–Fock (CPHF) equations. In analogy to the conventional non-local theory, the repeated solution of these equations for each degree of freedom can be avoided by using the z-vector method of Handy and Schaefer. Explicit equations are presented for the Pipek–Mezey localization. Test calculations on smaller organic molecules demonstrate that the local approximations introduce only minor changes of computed equilibrium structures.

The recently proposed explicitly correlated CCSD(T)-F12x (x = a,b) approximations [T. B. Adler, G. Knizia, and H.-J. Werner, J. Chem. Phys. 127, 221106 (2007)] are applied to compute equilibrium structures and harmonic as well as anharmonic vibrational frequencies for H(2)O, HCN, CO(2), CH(2)O, H(2)O(2), C(2)H(2), CH(2)NH, C(2)H(2)O, and the trans-isomer of 1,2-C(2)H(2)F(2). Using aug-cc-pVTZ basis sets, the CCSD(T)-F12a equilibrium geometries and harmonic vibrational frequencies are in very close agreement with CCSD(T)/aug-cc-pV5Z values. The anharmonic frequencies are evaluated using vibrational self-consistent field and vibrational configuration interaction methods based on automatically generated potential energy surfaces. The mean absolute deviation of the CCSD(T)-F12a/aug-cc-pVTZ anharmonic frequencies from experimental values amounts to only 4.0 cm(-1).

A numerically accurate implementation of the gauge-including atomic orbital method for the calculation of NMR shieldings in density functional theory (DFT) is presented. Results calculated by this method are compared with results of SCF and accurate coupled cluster calculations for eight small molecules. Three sets of DFT results, obtained using different exchange-correlation functionals, are further compared with each other, with the SCF data, and with experiment for a set of 10 somewhat larger organic molecules. The DFT values show a modest improvement compared to SCF theory. Potential computational savings using density functional theory are discussed.