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The log file (logfile) is generated during the calculation and flushed after (almost) each message that is sent to it by the program. Consequently, the user can inspect it and see what is going on without being delayed by potentially large system I/O buffers. Each message contains date and time of the message plus additional info.
A major part of the messages simply states the name of a procedure. Such messages are sent when the procedure is entered. During the SCF procedure, the SCF errors, which are a measure for non-self-consistency, are written at every cycle. In calculations where the geometry is changing (optimization, frequencies...) each set of new coordinates is sent to the log file (Cartesian, in angstrom and also Z-matrix, if a Z-matrix structure was provided in the input file). Other messages should be self-explanatory.
Be alert on error messages. Take them seriously: inspect the standard output carefully and try to understand what has gone wrong. Be also alert to warnings. They are not necessarily fatal but you should understand what they are about before being satisfied with the results of the calculation. Do not ignore them just because the program has not aborted: in some cases the program may not be able to determine whether or not you really want to do what appears to be wrong or suspicious. If you believe that the program displays erratic behavior, then the standard output file may contain more detailed information. Therefore, in such case save the complete standard output file, together with the logfile, in case we need these files for further analysis.
