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IRC run: please, explain how to get result

olgerdovich
olgerdovich@yandex.ru


Dear colleagues,

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
$CONTRL
 SCFTYP=RHF RUNTYP=IRC maxit=3000 ICHARG=0 MULT=1 MPLEVL = 2
$END
$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

$DATA
NH3
C1
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
$END

Please, help
Thanks in advance

[ This message was edited on Wed Aug 19 '15 at 3:38am by the author ]


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