Hi,

Can anyone help with using the right option for 'formula' keyword ? When to use 'onepoint' and 'twopoint'?

And what does this keyword mean?

I get different results using these options.

http://www.cse.scitech.ac.uk/ccg/software/chemshell/manual/force.html

Thank you,

Shreya

Hi Shreya,

They are different levels of approximation for calculating the Hessian by finite difference. A one point finite difference is calculated with a set of displacements starting from the initial geometry in each case, while a two point finite difference is calculated with points either side of the initial geometry. Using two points is more accurate, but also twice as expensive to calculate, so the choice depends on which of these factors is more important to you.

Best wishes,

Tom