CCL:G: Frequency calculation with fixed atoms.

 Sent to CCL by: Goedele Roos [groos(~)vub.ac.be]
 Hello,
 This is also of intrest for me...
 I often optimize with constraints but can I use this frozen geometry for example
 to calculate the frequencies of an H-bond?
 best regards, Goedele
 >Sent to CCL by: "Shobe, David" [dshobe]=[sud-chemieinc.com]
 >Ramon,
 >
 >When you use calcfc, you un-freeze the frozen atoms. So you'll have to
 freeze them again.  Something similar may be happenign with readfc.
 >
 >What exactly are you trying to do here?  Have you optimized the TS using the
 constraints (the frozen atoms) and now you want to remove these constraints and
 re-optimize?  Or do you want to keep the previously frozen atoms frozen?
 >
 >--David Shobe, Ph.D., M.L.S.
 >Süd-Chemie, Inc.
 >phone (502) 634-7409
 >fax (502) 634-7724
 >
 >Don't bother flaming me: I'm behind a firewall.
 >
 >
 >
 >-----Original Message-----
 >> From: owner-chemistry,,ccl.net [mailto:owner-chemistry,,ccl.net]
 >Sent: Wednesday, April 19, 2006 12:31 PM
 >To: Shobe, David
 >Subject: CCL:G: Frequency calculation with fixed atoms.
 >
 >Sent to CCL by: Ramon Crehuet [rcsqtc .. iiqab.csic.es] Dear CCLers,
 >    A few weeks ago I posted a question on how to calculate frequencies when
 a number of atoms are fixed. I got a solution, but now we have a related
 problem.
 >    I have used the ONIOM method in Gaussian 03 to optimize a transition
 state structure keeping several atoms fixed, but I have come up with a problem
 when reading a hessian. Firstly, I did a frequency calculation on the free
 atoms, including "-1" on frozen atoms an "0" on free atoms,
 and I obtained 1 negative eigenvalue:
 >
 >/                     1                      2                      3
 >                    A                      A                      A
 >Frequencies --   -41.1853                54.9227                70.3016
 >Red. masses --     6.2379                 7.0722                 3.5451
 >Frc consts  --     0.0062                 0.0126                 0.0103
 >IR Inten    --    17.8501                 1.2021                 1.2536
 >Atom AN      X      Y      Z        X      Y      Z        X      Y      Z
 >  1   8     0.00   0.00   0.00     0.00   0.00   0.00     0.00   0.00
 >0.00
 >  2   6     0.01  -0.02  -0.03     0.00   0.07   0.02     0.19   0.29
 >-0.13
 >  3   8     0.01  -0.01  -0.05     0.00   0.06   0.03     0.04   0.10
 >-0.01/
 >
 >
 >When I read the hessian from the checkpoint file (using the ReadFC keyword),
 in order to optimize the TS, I find that apparently the first point of the
 procedure does not use the previous hessian because now it has 5 negative
 eigenvalues:
 >
 >/Search for a saddle point.
 >Step number   1 out of a maximum of 696
 >All quantities printed in internal units (Hartrees-Bohrs-Radians) Swaping is
 turned off.
 >Second derivative matrix not updated -- first step.
 >    Eigenvalues ---   -0.00345  -0.00110  -0.00033  -0.00022  -0.00004
 >    Eigenvalues ---    0.00002   0.00004   0.00016   0.00017   0.00032
 >    Eigenvalues ---    0.00033   0.00036   0.00041   0.00047   0.00051/
 >
 >In addition, if I calculate the hessian analitically at the first point
 instead of reading it from a checkpoint file (using the CalcFC keyword) I obtain
 4 negative eigenvalues:
 >
 >/
 >Search for a saddle point.
 >Step number   1 out of a maximum of 696
 >All quantities printed in internal units (Hartrees-Bohrs-Radians)
 >Swaping is turned off.
 >Second derivative matrix not updated -- analytic derivatives used.
 >    Eigenvalues ---   -0.00053  -0.00020  -0.00014  -0.00002   0.00009
 >    Eigenvalues ---    0.00016   0.00017   0.00018   0.00025   0.00027
 >    Eigenvalues ---    0.00036   0.00039   0.00054   0.00066   0.00080/
 >
 >
 >It is clear that the hessian it reads is different from either the one
 >> from the checkpoint file or the one it calculates. Why are there these
 >differences, if they should all be the same? And, how should I carry out
 >the TS optimization successfully?
 >Thanks in advance,
 >
 >Ramonhttp://www.ccl.net/cgi-bin/ccl/send_ccl_messagehttp-:-//www.ccl.net/chemistry/sub_unsub.shtmlhttp-:-//www.ccl.net/spammers.txtThis
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 >Thank you.>
 >
 >
 >
 Drs. Goedele Roos
 Dienst Algemene Chemie (ALGC)
 Vrije Universiteit Brussel (VUB)
 Pleinlaan 2
 B-1050 Brussels
 Tel: 0032-2-629 35 16
 Fax: 0032-2-629 33 17