The ACES II/MOLCAS interface

For a long time, the ACES II program package was handicapped by its reliance on VMOL¹ and VDINT² for integrals and derivative-integrals, respectively. Not only are VMOL and VDINT slow by comparison to current competing packages, but they also suffer from the 256 basis function limitation when executed on 32-bit integer machines. Given these restrictions, it was nearly impossible to incorporate integral-direct methods into ACES II.

To remedy these problems, ACES II developers together with Dr. Roland Lindh developed an interface between ACES II and the MOLCAS program system. The primary goals of this collaboration were two-fold:

to allow the integral and integral-derivative modules in the MOLCAS program package (SEWARD, ALASKA, and MCKINLEY) to be used by the ACES II program system
to extend the post-Hartree-Fock capabilities that ACES II is well know for to the MOLCAS program system.

During July of 2000, Dr. Lindh visited the Gainesville group. He and his Gainesville counterparts (Ajith Perara, Anthony Yau, and Ken Wilson) designed and implemented a robust interface between the two program systems. The interface was done with minimal code sharing, and as a result, both program packages still exist as independent entities. Changes or further development of either package do not affect this interface. In our opinion, this is very desirable since both groups have different research interests and want to pursue them without interference.

To summarize, the following specific interfaces were added:

The MOLCAS integral program SEWARD was extended to generate VMOL-style integral files. Now, all the energy-based methods in ACES II can run with increased efficiency. This will not, however, rectify the 256 basis function limit on 32-bit integer machines.
Decoupled interfaces were developed to serve ACES II files to back-end programs, viz., JOBARC, GAMLAM, MOLINTS, etc. (all the post-HF direct access files). The ALASKA integral- derivative program requires contents from these files to generate gradients for ACES II wavefunctions. (Note that the ACES II program is unique in its gradient capabilities for RHF/ UHF/ROHF/QRHF CCSD, CCSDT, TD-CCSD, EOM-CCSD). This interface now allows the ALASKA code to pick up density matrices, effective Fock matrices, and two-particle density matrices in the AO basis suitable for contraction with derivative-integrals after sorting. The RHF, UHF, ROHF, and QRHF gradients are functional at the moment, and sorting routines are being developed by Roland Lindh. We can presently calculate correlated gradients.
ACES II analytical Hessian capabilities are currently limited to SCF and MBPT(2) (without symmetry). Since MOLCAS can now read ACES II files, ALASKA will be able to write the appropriate quantities to the MOINTS file, and the ACES II CPHF program will handle them painlessly, generating analytical second derivatives. The finishing touches on this interface are planned to be added in Fall 2000.

To remove the 256 basis function limit, Anthony Yau and Ajith Perera have begun modifying the existing index-based SCF algorithms to read MOLCAS-formated integral files. Meanwhile, Ken Wilson is planning to incorporate the direct-SCF capabilities of MOLCAS into ACES II. He will extend the existing RHF capabilities in MOLCAS (and in turn in ACES II) to UHF and ROHF.

Both MOLCAS developers and ACES II developers expect all the work to be completed by the end of Fall 2000.

Both groups have interchanged program package sources for local use and development free of charge.

This will be supported beginning with MOLCAS version 5.0. For ACES II, release is planned for late in the year 2000.

REFERENCES

¹: J. Almlof and P.R. Taylor; MOLECULE.
²: T. Helgaker, H.J. Aa. Jensen, P. Jorgensen, J. Olsen and P.R. Taylor; ABACUS.