Correlated ab initio calculations have been carried out with a parallel version of the PSGVB electronic structure code to obtain relative energetics of a number of conformations of the alanine tetrapeptide. The highest level of theory utilized, local MP2 with the cc-pVTZ(−f) correlation-consistent basis set, has previously been shown to provide accurate conformational energies in comparison with experiment for a data set of small molecules. Comparisons with published and new canonical MP2 calculations on the alanine dipeptide are made. Results for ten gas-phase tetrapeptide conformations and a β-sheet dipeptide dimer are compared with 20 different molecular mechanics force field parametrizations, providing the first assessment of the reliability of these models for systems larger than a dipeptide. Comparisons are made with the LMP2/cc-pVTZ(−f) results, which are taken as a benchmark for the tetrapeptides. Statistical summaries with regard to energetics and structure are produced for each force field, and a discussion of qualitative successes and failures is provided. The results display both the successes and limitations of the force fields studied and can be used as benchmark data in the development of new and improved force fields. In particular, comparisons of hydrogen-bonding energetics as a function of geometry suggest that future force fields will need to employ a representation for electrostatics that goes beyond the use of atom-centered partial charges.

We present an outline of the parallel implementation of our pseudospectral electronic structure program, Jaguar, including the algorithm and timings for the Hartree–Fock and analytic gradient portions of the program. We also present the parallel algorithm and timings for our Lanczos eigenvector refinement code and demonstrate that its performance is superior to the ScaLAPACK diagonalization routines. The overall efficiency of our code increases as the size of the calculation is increased, demonstrating actual as well as theoretical scalability. For our largest test system, alanine pentapeptide [818 basis functions in the cc‐pVTZ(‐f) basis set], our Fock matrix assembly procedure has an efficiency of nearly 90% on a 16‐processor SP2 partition. The SCF portion for this case (including eigenvector refinement) has an overall efficiency of 87% on a partition of 8 processors and 74% on a partition of 16 processors. Finally, our parallel gradient calculations have a parallel efficiency of 84% on 8 processors for porphine (430 basis functions). © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1017–1029, 1998

Theoretical calculations of ozone photoabsorption and photoemission in the Hartley band are presented. The potential energy surfaces of Sheppard and Walker [J. Chem. Phys. 78, 7191 (1983)] and of Barbe et al. [J. Mol. Spectrosc. 49, 171 (1974)] are used for the excited (1B2) and ground (1A1) states, respectively. In contrast with several recent studies, large amplitude motion in the symmetric and asymmetric stretch coordinates is explicitly included. Qualitative agreement is obtained with the experimental emission spectrum at 266 nm, although some discrepancies persists, using either the original Sheppard–Walker surface or various modifications thereof. Moreover, the calculations do not reproduce the experimentally observed structure atop the Hartley absorption band, eliminating some possibilities for the origin of this structure. The photofragmentation dynamics was computed by numerical integration of the time-dependent Schrödinger equation on a two-dimensional grid, and spectral observables were recovered via Fourier transform.