The original copyright notice of MOPAC7:

C
C         Notice of Public Domain nature of MOPAC
C
C      'This computer program is a work of the United States
C       Government and as such is not subject to protection by
C       copyright (17 U.S.C. # 105.)  Any person who fraudulently
C       places a copyright notice or does any other act contrary
C       to the provisions of 17 U.S. Code 506(c) shall be subject
C       to the penalties provided therein.  This notice shall not
C       be altered or removed from this software and is to be on
C       all reproductions.'
C

Changes made to MOPAC7 code in this work may be used/distributed under the
same rules as the original MOPAC7 package; this work is in Public Domain.