Hello,
I was trying to calculate the zwitterion substrate's minimum value with Gaussian 09.
Tried several calculations but, their results showed that one imaginary value.
I don't think this substrate is transition state of something so, I thought the results should not have an imaginary value.
How can I solve this problem?
Is there any special way to calculate zwitterion? (restricted or unrestricted method/singlet or other etc.)
Or any previous steps that I need to go over?
Thank you.
Dear Lucy Ryou,
Depending on your molecule, you must adapt the calculation (charge, multiplicity and of course restricted and unrestricted). If your molecule is a zwitterion like a zwitterionic form of an amino-acid, you should use a restricted method with 0 as charge and multiplicity as 1. One method to avoid an imaginary frequency is to know what causes this imaginary frequency. For this purpose, I suggest you to use a software like Molden to know what are the atoms involved in this imaginary frequency. Then you can move the coordinates of these atoms and make another optimization with the new coordinates. By experience, I often meet imaginary frequency with methyl group when I perform a molecular dynamic calculation before optimization (that helps me to give a relatively good structure to optimize by quantum calculation). Don't hesitate to contact me if you cannot solve your problem.
First think to do is to animate the imaginary frequency in order to see the movement that it describes. What is the value of this imaginary freq? is it low? It could be just a rotation. Then displace your structure following this imaginary frequency (for instance you can use Chemcrafts or Gaussview to do that), and take the new (displaced) structure as starting point for a new optimization. Some thimes these spureous frequencies are difficult to eliminate and you must repeat this operation several times.
Studying this article can help you in the field of zwitterion.
"Computational Modeling and Simulation of CO2 Capture by Aqueous
Amines / DOI: 10.1021/acs.chemrev.6b00662"
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