Searching....

# B.11.1 Overview

(February 4, 2022)

Q-Chem combines the Head-Gordon–Pople (HGP) method439 and the COLD prism method22 for Hartree-Fock gradient and Hessian evaluations. All two-electron four-center integrals are classified according to their angular momentum types and degrees of contraction. For each type of integrals, the program chooses one with a lower cost. In practice, the HGP method is chosen for most integral classes in a gradient or Hessian calculation, and thus it dominates the total CPU time.

Recently the HGP codes within Q-Chem were completely rewritten for the evaluation of the P II${}^{x}$ P term in the gradient evaluation, and the P II${}^{xy}$ P term in the Hessian evaluation. Our emphasis is to improve code efficiency by reducing cache misses rather than by reducing FLOP counts. Some timing results from a Hartree-Fock calculation on azt are shown below.