PQS is a general purpose quantum chemistry program. Its roots go back to the first ab initio gradient program developed in Professor Peter Pulay's group but now it is developed and distributed commercially by Parallel Quantum Solutions. There is a reduction in cost for academic users and a site license. Its strong points are geometry optimization, NMR chemical shift calculations, and large MP2 calculations, and high parallel efficiency on computing clusters. It includes many other capabilities including Density functional theory, the semiempirical methods, MINDO/3, MNDO, AM1 and PM3, Molecular mechanics using the SYBYL 5.0 Force Field, the quantum mechanics/molecular mechanics mixed method using the ONIOM method, natural bond orbital (NBO) analysis and COSMO solvation models. Recently, a highly efficient parallel CCSD(T) code for closed shell systems has been developed. This code includes many other post Hartree–Fock methods: MP2, MP3, MP4, CISD, CEPA, QCISD and so on. History The origin of PQS program was developed by Meyer and Pulay in the late 1960s. They both were at the Max-Planck Institute for Physics and Astrophysics in Munich when they began to write a new ab initio program. The main purpose was to establish new ab initio techniques. Pulay and Meyer had slightly different interests. Pulay was interested in implementing gradient geometry optimization, analytical energy derivatives (force), and force constant calculations via the numerical differentiation of analytical forces, while Meyer enthused about the coupled-electron pair approximation (CEPA), spin density calculation, and extremely accurate correlation methods like pseudonatural orbital-configuration interaction (PNO-CI). At that time, analytical gradients were limited to closed-shell Hartree-Fock wavefunctions. However, they were able to do it for unrestricted (UHF) and restricted open-shell (ROHF) methods in 1970. The first version of the code was completed in 1969 at Max-Planck institute and University of Stuttgart. Then, Meyer named it “MOLPRO” and used Gaussian lobe basis sets. In the 1970s, the current version of MOLPRO added a number of advanced methods such as multiconfiguration self-consistent field (MC-SCF) and internally contracted multireference configuration interaction (MR-CI). Simultaneously, in the 1980s, MOLPRO was extended and mostly rewritten by Hans-Joachim Werner, Peter Knowles and Meyer's coworkers. Meanwhile, in 1976, Pulay was visiting Boggs at the University of Texas, Austin and Schaefer at the University of California. They wrote a new program called TEXAS based on the original MOLPRO and replaced Gaussian lobe functions with the standard Gaussian functions. TEXAS emphasized large molecules, SCF convergence, geometry optimization techniques, and