# Gaussian simultenous scan with two variables

I am trying to rigid scan two bond distances simultaneously in Gaussian 09/16. For example, I have taken a water molecule and started a rigid scan. Below is my input.

# b3lyp/6-31+g(d,p) scan

Title Card Required

0 1
O
H                  1            B1
H                  1            B2    2            A1

A1           109.50000006

B1 0.96 S 20 0.1
B2 0.96 S 20 0.1



The output scan plot is this:

It is a 3D plot of 441 (21 steps for H1 atom * 21 steps for H3 atom) grid points. I want both the H atoms to move simultaneously, so I get a 2D plot of only 21 points. Below is the type of plot I want for scanning 2 H atoms. Any help with the input?

• I'm not sure, but it's certainly easy enough to try: use a single variable for both bond lengths. Mar 25, 2023 at 1:12
• That seems fine, though. However, I would be thankful to you if you please share any sample input.
– Pro
Mar 27, 2023 at 9:29
• In your z-matrix substitute B2 with B1 and delete the last line before the mandatory blank line. Mar 28, 2023 at 16:56
• it works, thanks
– Pro
Mar 30, 2023 at 14:32

From the obvious hints @Martin provided, it became easy. We can simultaneously scan only one coordinate. So, for this particular problem the input should be as follows,

# hf/3-21g scan

Title Card Required

0 1
O
H                  1            B1
H                  1            B1    2            A1

A1           109.50000006

B1 0.96 S 20 0.1


Explanation: Since bond the bond lengths of OH are same, one has to put only B1 parameter in the z-matrix and vary that parameter. But, for a special case, if $$OH_1\neq OH_2$$ then this will not work. I will look for more general case and post that as update here.

• In my opinion don't bother looking for a more general case. Gaussian interface is not flexible enough for that. There is possibly an easier way to generate a series of xyz coordinates programmatically with python of so and run single points on these. For future questions like these, I recommend looking at Matter Modeling. Mar 30, 2023 at 16:46
• Oh... I forgot, you might have luck with GIC. But in all honesty, learning python is probably easier... Mar 30, 2023 at 16:48