28 The FCIQMC program (FCIQMC)

The FCIQMC program exists through an interface to the NECI codebase, which is actively developed in the group of A. Alavi, and has been integrated into the MOLPRO code. The FCIQMC method is a recently introduced stochastic method which can calculate in principle FCI-quality energies for small to medium-sized molecules. The method should also be suitable for large-scale parallel application across distributed memory architecture. However, it should be noted that since the method relies on random number strings, the exact path of the simulation will change with number of MPI tasks used, although converged results should agree. Since the method is new, the code is also somewhat developmental, and updates and improvements are to be expected. Any user hoping to use the FCIQMC package for a study is welcome to get in contact directly with A. Alavi (asa10@cam.ac.uk), and the lead developer of the code, G. Booth (ghb24@cam.ac.uk), for technical help, advice and access to undocumented options.

It is also highly recommended to invoke the command-line option –no-helper-server, which will ensure that when running in parallel, all MPI processes are utilized. Multiple files are created in addition to the standard output, and dynamic adjustment to the calculation can only take place from the working directory. Therefore it may be useful to assign a convenient working directory from the command-line when MOLPRO is run. This can be done with the -d flag, with -d . setting the current directory as the working directory. Also, since the printing to the output file is buffered, it may be better to view the running simulation from the .xml file.

molpro@molpro.net 2020-04-18