[thanq]

Hello!
I need some help on how to do partial geometry optimization using the model B3LYP/6-31G** and also calculate the reaction barrier and change in enthalpy for the reaction:
Cl- + Et-Br = Et-Cl + Br-
Et - ethane
I need to make two Z-matrices where in the first one the chlorine attacks carbon and bromine is the leaving group and second is the other way around. I've encountered two different methods: opt=Z-matrix where I use variables e.g r - bond length; a - angle. The other one is opt=ModRedundant which I do not understand completely.
So do you guys have any advice what to do? Has anyone used ModRedundant and explain it to me. I have added my current z-matrix. As I don't have gaussian at home I can't check whether it is correct or not.

#P B3LYP/6-31G** opt=Z-Matrix

Cl atakeerib

0 1
C1
C2 C1 1.5
Br C1 1.94 C2 90.0
Cl C1 r1   Br a1 C2 90.0
H1 C1 1.0  C2 110.0 Br 90.0
H2 C2 1.0  C1 110.0 H1 0.0
H3 C2 1.0  C1 110.0 H1 120.0
H4 C2 1.0  C1 110.0 H1 -120.0
H5 C1 1.0  C2 110.0 H1 180.0

r1=3.8 S 5 -0,4
a1=180.0 F

In this matrix the aim was to line Cl and Br on the same line (recommended by the professor) and r1 is variable closing in to the carbon.

[Irlanur]

I dont know Guassian (I am only familiar with TURBOMOLE), but

6-31G**

is usually so bad that it's not even worth to be applied for anything "serious". (I know it's used a lot anyway).