MOLPRO's execution is controlled by user-prepared data; if datafile is not given on the command line, the data is read from standard input, and program results go to standard output. Otherwise, data is taken from datafile, and the output is written to a file whose name is generated from datafile by removing any trailing suffix, and appending .out. If the output file already exists, then the old file is appended to the same name with suffix .out_1, and then deleted. This provides a mechanism for saving old output files from overwriting. Note that the above behaviour can be modified with the -o or -s options.