MOLCAS manual: 2.2 Alternative versions of Molcas Before Molcas 8.6, it was possible to have several installations of Molcas, which can be selected by exporting an enviroment variable MOLCAS, e.g. export MOLCAS=/home/molcas/. This option is still available. Molcas 8.6 provides another way to use several versions of Molcas at the same time. A special file molcas.alt contains the list of available Molcas installations. This file is automatically generated, and has a simple format: an alias and the location of the tree, where bin/ directory contains the executables. In order to use alternative versions of Molcas, user can use q command molcas -use=ALIAS input.file. For instance, molcas -use=86_prebuilt input.file. It is also possible to use EMIL command inside the input file: »USE 86_prebuilt.