Gaussian Keywords: #

The route section of a Gaussian job is initiated by a pound sign (#) as the first non-blank character of a line. The remainder of the section is in free-field format. For most jobs, all of the information can be placed on this first line, but overflow to other lines (which may but need not begin with a # symbol) is permissible. The route section must be terminated by a blank line.

If no keywords are present in the route section, the calculation defaults to HF/STO-3G SP.

How much output is generated during a geometry optimization (and actually all other types of Gaussian calculations as well) depends on the beginning of the keyword line:

# (#N)      will provide normal output (normal is the default) #P            will provide somewhat more detailed output. This includes messages at the beginning and end of each link giving assorted                 machine-dependent information (inclusing execution timing data), as well as convergence information in the SCF. #T            will provide somewhat less output (terse). Output is reduced to essential information and results.

Reference : Electronic Structure Calculations in Gaussian