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Hi Jim,
The behaviour of your calculations is normal, at
least in Gaussian.
G03, by default, does not change the symmetry point
group of the input molecular coordinates. When you start a calculation with a
perfectly linear NaCN (or OCS-) molecule, whose point group is Cinfv, the
optimization will never reach the bent geometry, of lower Cs symmetry. In order
to optimize a bent (or T shape) geometry you should start from a bent
molecule. Any of these optimizations (linear or bent) will finish in a
stationary point in the PES (potential energy surface), which could be a local
minimum or a saddle point, depending on number of negative frequencies in the
frequency calculation: none means local minimum in the PEs, one means transition
state (first order saddle point), and more a higher order saddle
point.
If you want to know if your molecule is linear or
bent (which includes T shape), you don't really need any knowledge of the
experimental geometry, at least with these small, triatomic systems. You can
optimize both a linear and a bent (it does not matter the input angle, it
will eventually reach the experimental) molecule, and then run a frequency
calculation in both. The might be both local minimums, and the experimental
geometry (at least in the gas phase) should correspond to the one whose energy
is lower, which is the global minimum in the PES. Or one of them may be a
transition state and the other one a local minimum.
For more complicated systems, the calculation is
(much) faster if you start from a high symmetry geometry, and check the
frequencies. If there are any negative ones, decrease the symmetry (as
indicated by the normal mode corresponding to the negative frequency) and
optimize again.
In the case of OCS-, the negative frequency for the
linear molecule indicates that the geometry has been optimized to the nearest
stationary point of linear symmetry, which happens to be a transition state. In
fact, the Gaussian output indicates that the negative frequency corresponds to a
movement of the atoms away from linearity. On the other hand, the bent OCS-
optimizes to a local minimum, with an angle of about 136º and no negative
frequencies.
The linear OCS could have been another local
minimum, but much higher in energy than the bent one.
I hope this helps you a little bit
Best regards
Pablo
----- Original Message -----
Sent: Friday, January 26, 2007 6:39
AM
Subject: CCL:G: Gaussian 03 -
optimization (linear, non linear) - negative frequency
Dear all
I have a problem concerning
optimization of molecules of more than 2 atoms.
I am using Gaussian 03 to
optimize and perform a frequency calculation of molecules such as for example
NaCN. NaCN is a T shape molecule and has a non-linear
geometry.
If I input a linear initial geometry and perform an
optimization, will the molecule (which is actually non-linear) converge to a
non-linear geometry
I have perform an optimization of linear NaCN and
the stationary point was found.The optimized geometry remains linear. But when
I perform optimization of
non-linear NaCN with initial angle 120, the
required geometry (T shaped) was obtained.
My question is that must
one have a knowledge of the geometry of a molecule under study in its ground
state to perform optimization. If so what is the purpose of optimizing. Can an
initial linear structure input converged to an optimized structure which is
actually non-linear
Can Gaussian 03 predict the structure of a molecule for
which the geometry is unknown. If so how?
I know that Gaussian 03
required an approximate initial geometry. Is there a way
from theory to
determine whether the molecule in question is linear, bent or even a T
shape.
For example OCS molecule is linear. If one is going to calculate
the adiabatic electron affinity, one has to optimise the anion OCS- which I
did with a linear
geometry just like with OCS. The optimize anion OCS-
remains linear. But I have found from many publications that anion OCS- is
actually a bent shape molecules.Again I perform an optimization of OCS-
but this time with a bent shape, angle 120. This optimized geometry with angle
136.7 coincide with the publications. The angle has change significantly
from angle 120 to 136.7.
Why the initial linear geometry of anion OCS-
did not converge to the optimised geometry with angle 136.7
note that
the electron affinity computed with linear anion OCS- and the bent shaped
anion OCS- are totally different. The one which coincide with the experimental
value is the bent shaped as required.
I perform a frequency calculation
with the optimized geometry of the linear anion
OCS- and obtain
negative frequency (saddle point which specifies a transition state). Does
a negative frequency mean the optimized geometry has not actually
been optimized. Does in general a negative frequency is an indication that
the initial geometry input is not correct and hence must alter the initial
geometry just like I did with the bent shape.
Thanking you
all
Best regards
Jim
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