To get some practice in tracing reaction path from TS to products, I tried some IRC-runs on one of the simplest model reaction, ammonia inversion, but I faced with problem even in this very simple case. Unfortunately, manual gave me no hints, so I ask you to instruct me how to do it in right way.
First, I optimized flat NH3-molecule (MP2/6-311++G**) by freezing dihedral at 180° in z-matrix coordinate input. It gave me flat (D3h) NH3 with r(N-H) = 0.99890 A with one imaginary frequency (850.33 I), six rotational/translation freedom degrees (five not exceeding 18 cm-1 and one rather high, 125 cm-1), and five strong vibrations (1590...3910 cm-1)
Second, I put this geometry as starting and start IRC run by changing RUNTYP for IRC, adding "$irc forwrd=.t. npoint=1000 MXOPt = 1000 $end" string and $HESS group from flat molecule optimization run, leaving all the other parameters constant, and problems began
I was forced to add MXOPT explicitly to defaults since otherwise firefly stopped at the second IRC-iteration (TOO MANY STEPS TAKEN IN CONSTRAINED OPTIMIZATION). Being thus unleashed, Firefly found another reason to stop at the same second iteration after 226 steps, claiming:
RADIUS IN CIRCLE OPTIMIZATION 0.1860667 DEVIATES SIGNIFICANTLY FROM CONSTRAIN CONDITION 0.1500000
IT IS POSSIBLE THAT THE NEXT IRC POINT IS CLOSE TO A MINIMUM
(actually, it was still very far from pyramidal ammonia)
adding "pace=linear stride=0.10" to $IRC group rapidly (just on POINT 1 ON THE REACTION PATH) resulted in
YOU ARE NO LONGER ON A STEEPEST DESCENT PATH!
THIS IRC SHOULD BE RUN WITH SMALLER STEPS AND/OR A BETTER INTEGRATION METHOD.
and lowering STRIDE to stride=0.01 and adding elbow=180 changed nothing, as well as switching to pace=AMPC4 or pace = RK4 .
Here is one of the tested inputs:
$BASIS GBASIS=N311 NGAUSS=6 NDFUNC=1 NPFUNC=1 DIFFSP=.T. DIFFS=.T. $END
SCFTYP=RHF RUNTYP=IRC maxit=3000 ICHARG=0 MULT=1 MPLEVL = 2
$CONTRL INTTYP=HONDO, ICUT=11, ITOL=30 $END
$SCF dirscf=.t. FDIFF=.FALSE. DIIS = .T. SOSCF = .F. $END
$STATPT OPTTOL=0.00001 NSTEP=100 HSSEND=.TRUE. $END
$irc forwrd=.t. pace=RK4 stride=0.01 npoint=1000 MXOPt = 1000 $end
NITROGEN 7.0 0.000000175 -0.000000255 -0.000000001
HYDROGEN 1.0 0.865069400 0.499451188 0.000000006
HYDROGEN 1.0 -0.865070520 0.499447858 0.000000006
HYDROGEN 1.0 -0.000001304 -0.998895509 0.000000006
Thanks in advance
[ This message was edited on Wed Aug 19 '15 at 3:38am by the author ]