Throws an error if you haven't set your $MOPAC_DIR environment variable.
Throws an error if you haven't set your $MOPAC_DIR environment variable.

Previously it would fail with a more cryptic error message if this wasn't set.
comocheng/RMG-Py,chatelak/RMG-Py,pierrelb/RMG-Py,nyee/RMG-Py,faribas/RMG-Py,KEHANG/RMG-Py,chatelak/RMG-Py,nickvandewiele/RMG-Py,pierrelb/RMG-Py,nyee/RMG-Py,nickvandewiele/RMG-Py,enochd/RMG-Py,enochd/RMG-Py,faribas/RMG-Py,comocheng/RMG-Py,KEHANG/RMG-Py