If MOPAC has been compiled so that the special SCF convergers can be used, then the memory demand will be higher than if they are not allowed. These convergers are not needed for well-behaved systems, and therefore more memory is needed than is used. To reduce the memory demand for a run, add UNSAFE to the keyword line. A side effect of this command is that some systems will fail to generate an SCF, and the run may be wasted as a result. Do not use UNSAFE unless there is a need to conserve memory resources.