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4
GENERAL PROGRAM STRUCTURE
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most circumstances the program defaults are appropriate, and the user does not have to specify
the records.
After a restart this logic will still work correctly if the number and sequence of SCF and MCSCF
commands is kept unchanged. Thus, if you want to skip certain parts of the input after a restart,
it is recommended to use IF / ENDIF structures or the GOTO command rather than to delete or
comment certain commands. If for some reason this is not possible, the START and ORBITAL
directives can be used to specify explicitely the records to be used.
In general we recommend the use of program defaults whenever possible, since this minimizes
the probability of input errors and frustration!
After completion of each program step, M OLPRO prints a summary of the records on each file.
4.4
Restart
Information from the permanent files is automatically recovered in subsequent calculations.
This can be controlled using the RESTART directive.
4.5
Data set manipulation
It is possible to truncate files and rename or copy records using the DATA command. Several standard matrix operations can be performed with MATROP, e.g., printing records, linearly
combining or multiplying matrices, or forming the trace of a product of two matrices.
4.6
Memory allocation
M OLPRO allocates memory dynamically as required by the user on the MEMORY card. Thus it is
not necessary to maintain different versions of the program with different memory sizes. If the
MEMORY command is omitted, the program will use a default memory size, which depends on
the hardware used and how the program was installed. Note that, on Unix machines, the default
memory can be set on the molpro command line using the flag -m.
4.7
Multiple passes through the input
It is possible to perform loops over parts of the input using DO loops, very much as in FORTRAN
programs. DO loops may be nested to any reasonable depth. This can be conveniently used, for
instance, to compute automatically whole potential energy surfaces.
4.8
Symmetry
M OLPRO can use Abelian point group symmetry only. For molecules with degenerate symmetry, an Abelian subgroup must be used — e.g., C2v or D2h for linear molecules. The symmetry
group which is used is defined in the integral input by combinations of the symmetry elements
x, y, and z, which specify which coordinate axes change sign under the corresponding generating symmetry operation. It is usually wise to choose z to be the unique axis where appropriate
(essential for C2 and C2h ). The possibilities in this case are shown in Table 1.
Normally, M OLPRO determines the symmetry automatically, and rotates and translates the
molecule accordingly. However, explicit symmetry specification is sometimes useful to fix the
orientation of the molecule or to use lower symmetries.