CCL:G: problem with geometry optimization in g09

[Thomas Manz <thomasamanz . gmail.com>]

 Sent to CCL by: Thomas Manz [thomasamanz!A!gmail.com]
 Hi Walter,
 An sto-3g or 3-21G basis set is extremely small, so unless your system
 contains more than a few hundred atoms it's probably not an
 appropriate choice.
 The 6-311++G** or another triple zeta basis set with polarization and
 diffuse functions would be a better choice if your system contains
 fewer than about 200 atoms.
 Generally speaking, you would be better off using a correlated
 electron method like density functional theory (e.g., B3LYP) than a
 semi-empirical or hartree-fock calculation.
 If your system contains hundreds of atoms, consider using a pure DFT
 functional like PW91PW91 with a density fitting basis set (e.g.,
 PW91PW91/6-311++G**/auto).
 Unless there is a compelling reason to keep the Fe atoms fixed, why
 not relax the whole structure?
 Route line of the following type might be suitable:
 # opt B3LYP/6-311++G** scf=(fermi,maxcycle=400)
 You can find further instructions for how to perform a calculation of
 this type in the Gaussian manual at www.gaussian.com.
 Sincerely,
 Tom
 On Wed, Apr 20, 2011 at 10:32 AM, Walter Ca on wcanon[]ciq.uchile.cl
 <owner-chemistry^_^ccl.net> wrote:
 >
 > Sent to CCL by: "Walter  Ca  on" [wcanon ~
 ciq.uchile.cl]
 > Hi, my name is Walter and we have the following problem.
 > we are trying to perform a partial geometry optimization of some hydrogens
 atoms using g09, in a system with 3 Fe(III) atoms, in wich those 3 Fe(III) atoms
 are fixed. the system have charge +3. I have try from semi-empirical methods to
 UHF with some simple basis set like sto-3g or 3-21G, because i only want to
 optimized hydrogens atoms.
 > Can anyone help me to try to fix this problem.
 > Thanks
 > Walter>      http://www.ccl.net/cgi-bin/ccl/send_ccl_message>;  
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 >
 >