3.3.4. SEWARD — An Integral Generation Program

An ab initio calculation always requires integrals. In the Molcas suite of programs, this function is supplied by the SEWARD module. SEWARD computes the one- and two-electron integrals for the molecule and basis set specified in the input to the program GATEWAY, which should be run before SEWARD. SEWARD can also be used to perform some property expectation calculations on the isolated molecule. The module is also used as an input parser for the reaction field and numerical quadrature parameters.

We commence our tutorial by calculating the integrals for a water molecule. The input is given in Block Each Molcas module identifies input from a file by the name of the module. In the case of SEWARD, the program starts with the label &SEWARD, which is the first statement in the file shown below.

In normal cases no input is required for SEWARD, so the following input is optional. The first keyword used is TITLe. Only the first line of the title is printed in the output. The first title line is also saved in the integral file and appears in any subsequent programs that use the integrals calculated by SEWARD.

Block Sample input requesting the SEWARD module to calculate the integrals for water in \(C_{2v}\) symmetry.
Water - A Tutorial. The integrals of water are calculated using C2v symmetry

In more complicated cases more input may be needed, to specify certain types of integrals, that use of Cholesky decomposition techniques (CHOLesky keyword), etc. We refer to the specific sections of the User’s Guide for more information. The output from a SEWARD calculation is small and contains in principle only a list of the different types of integrals that are computed.

The integrals produced by the SEWARD module are stored in two files in the working directory. They are ascribed the FORTRAN names ONEINT and ORDINT which are automatically symbolically linked by the Molcas script to the file names $Project.OneInt and $Project.OrdInt, respectively or more specifically, in our case, water.OneInt and water.OrdInt, respectively. The default name for each symbolical name is contained in the corresponding program files of the directory $MOLCAS/shell. The ONEINT file contains the one-electron integrals. The ORDINT contains the ordered and packed two-electron integrals. Both files are used by later Molcas program modules.