My goal is to calculate activation energy of proton dissociation. Therefore; I planned to calculate energy of reactant complex and transition structure, and then subtract reactant energy from transition structure energy.
For the first step, I'm trying to optimize transition structure in the proton dissociation reaction.
Reactant complex is molecule + water.
But before going that calculation, I want to know how to optimize transition structure first, so I start with simple example in the website :
http://www.gaussian.com/g_whitepap/qst2.htm
I made my input file according to instructions in above site.
This is what I wrote for my input file
#T RHF/6-31G(d) Opt=(QST2,AddRedundant)
SiH2+H2>SiH4 Reactants
0 1
Si
X 1 1.0
H 1 1.48 2 55.0
H 1 1.48 2 55.0 3 180.0
H 1 R 2 A1 3 90.0
H 1 R 5 A2 2 180.0
R 2.0
A1 80.0
A2 22.0
SiH2+H2>SiH4 Products
0 1
Si
X 1 1.0
H 1 1.48 2 55.0
H 1 1.48 2 55.0 3 180.0
H 1 R 2 A1 3 90.0
H 1 R 5 A2 2 180.0
R 1.48
A1 125.2
A2 109.5
4 5
But it causes error when I start calculation
Error message is like this :
---------------------------------------
#T RHF/6-31G(d) Opt=(QST2,AddRedundant)
---------------------------------------
----------------------
SiH2+H2>SiH4 Reactants
----------------------
Symbolic Z-matrix:
Charge = 0 Multiplicity = 1
Si
X 1 1.
H 1 1.48 2 55.
H 1 1.48 2 55. 3 180. 0
H 1 R 2 A1 3 90. 0
H 1 R 5 A2 2 180. 0
Variables:
R 2.
A1 80.
A2 22.
The following ModRedundant input section has been read:
Invalid number of scan points.
SiH2+H2>SiH4 Products
==================================================
However; when I calculate after removing AddRedundant and last line '4 5'. It works, though result is not like transition structure.
#T RHF/6-31G(d) Opt=(QST2)
SiH2+H2>SiH4 Reactants
0 1
Si
X 1 1.0
H 1 1.48 2 55.0
H 1 1.48 2 55.0 3 180.0
H 1 R 2 A1 3 90.0
H 1 R 5 A2 2 180.0
R 2.0
A1 80.0
A2 22.0
SiH2+H2>SiH4 Products
0 1
Si
X 1 1.0
H 1 1.48 2 55.0
H 1 1.48 2 55.0 3 180.0
H 1 R 2 A1 3 90.0
H 1 R 5 A2 2 180.0
R 1.48
A1 125.2
A2 109.5
To recap, My question is how to use qst to get transition state. I guess I have some problem in AddRedundant option, but I don't know what it is and how to use it
and,, If anyone know other method to calculate activation energy of proton dissociation, please let me know.
Thanks!
Hello Hyun,
I have added an example of input for QTS2 calculus. Be carful with contruction, atoms must be at same order. Also using connector will help you.
I hope it helps you
The problem with your input is the fact that you use the addredundant option, which tells gaussian to look for redundant coordinates specification after variables section. However, your first geometry does not have any redundant coordinates. So gaussian thinks that the line "SiH2+H2>SiH4 Products" is the redundant coordinates specification and fails, since it cannot understand it.
To make it work you have to specify your reduncant coordinates (4 5 in your example) after each geometry. From gaussian website:
ModRedundant
Add, delete or modify redundant internal coordinate definitions (including scan and constraint information) before performing the calculation. This option requires a separate input section following the geometry specification. When used in conjunction with QST2 or QST3, a ModRedundant input section must follow each geometry specification. AddRedundant is synonymous with ModRedundant.
Thanks for your answers!
I removed addredundant option, Joaquim's file also doesn't have the option, and run the calculation.
Sometimes I can get transition structure, but when I change parameters little bit; like distance between two molecules, angles, and dihedral angles. Error occurs.
I just want to optimize product complex structure little bit more, but this keeps making errors.
I wish if you know what I meant, please give me some advice how to handle such an error.
Thanks again :)
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