! EXAM06.
!    1-A-1 CH2    MCSCF methylene geometry optimization.
!    The two configuration ansatz is the same as used in
!    the fourth example.
!
!    The optimization is done in internal coordinates,
!    as NZVAR is non-zero.   Since a explicit $ZMAT is
!    given, these are used for the internal coordinates,
!    rather than those used to enter the molecule in
!    the $DATA.  (Careful examination of this trivial
!    triatomic's input shows that $ZMAT is equivalent
!    to $DATA in this case.  You would normally give
!    $ZMAT only if it is somehow different.)
!
!    This job tests the MCSCF wavefunction and gradient.
!
!    At the initial geometry:
!    The initial energy is -37.187342653,
!    the FINAL E= -37.2562020559 after 14 iterations,
!    the RMS gradient is 0.0196185.
!
!    After 4 steps,
!    FINAL E= -37.2581791690, RMS gradient=0.0000012,
!    r(CH)=1.1243353, ang(HCH)=98.8170741
!
 $CONTRL SCFTYP=MCSCF RUNTYP=OPTIMIZE NZVAR=3 COORD=ZMT $END
 $SYSTEM TIMLIM=1 $END
 $BASIS  GBASIS=STO NGAUSS=2 $END
 $DATA
Methylene...1-A-1 state...MCSCF/STO-2G
Cnv  2

C
H 1 rCH
H 1 rCH 2 aHOH

rCH=1.09
aHOH=99.0
 $END
 $ZMAT   IZMAT(1)=1,1,2,   1,1,3,   2,2,1,3  $END
!
! Normally one starts a MCSCF run with converged SCF
! orbitals, as Huckel orbitals normally do not converge.
! Even if they do converge, the extra iterations are
! very expensive, so use MOREAD for your runs!
!
 $GUESS  GUESS=HUCKEL $END
!
! two active electrons in two active orbitals.
! The ground 3-B-1 state is of different symmetry so we
! need only solve for the lowest A-1 symmetry root.
!
 $DET    NCORE=3 NACT=2 NELS=2 STSYM=A1 NSTATE=1 $END