vibrational spectroscopy-related calculations. From 1982 onward, the program was further developed at the University of Arkansas. The primary significant expansion was the usage of a few new electron correlation methods by Saebo and a first- order MC-SCF program by Hamilton. A critical option was the implementation of the first practical gauge-invariant atomic orbital (GIAO) NMR program by Wolinski, who additionally included a highly efficient integral package. Bofill executed an unhindered natural orbital-complete active space (UNO-CAS) program including analytical gradients; this is a minimal-cost alternative to MC-SCF and works just as well in most cases. TEXAS was initially parallelized in 1995–1996 on a cluster of 10 IBM RS6000 workstations. In 1996, Baker joined Pulay and, around the same time, Intel brought out the Pentium Pro, a processor for PC that was competitive with low-end workstations and less costly by around an order of magnitude. Understanding the capability of this improvement for computational chemistry, PQS was formed and a SBIR grant application was submitted in July 1997 for the commercial development of PC clusters for parallel ab initio calculations. In the meantime, the Pulay group, financed by a National Science Foundation grant, set about constructing a Linux cluster using 300 MHz Pentium II processors. By a fortunate circumstance, a few capable and PC proficient graduate students were available, specifically Magyarfalvi and Shirel. The PC cluster was a complete success, and significantly outperformed the IBM Workstation cluster that was the group's computational mainstay at a small amount of its expense. The PQS programming was demonstrated on the TEXAS code and parts of it, chiefly the NMR code, were authorized to PQS from the University of Arkansas. Much of the code was widely changed to comply with the twin points of (a) having all major functionality fully parallel; and (b) having the capacity to routinely perform calculations on extensive systems. They aimed primarily for a modest level of parallelism (from 8 to 32 CPUs), as this is the most widely recognized size for an individual or group resource. Indeed, even on very large clusters it is normal for any given user to be allocated only a percentage of the available processors. Features The basis capabilities in high-level correlated energies for PQS ab initio v. 4.0 include MP3, MP4, CID, CISD, CEPA-0, CEPA-2, QCISD, QCISD(T), CCD, CCSD and CCSD(T) wavefunctions; enforced geometry optimization (used, among other things, to simulate the results of Atomic Force Microscopy