how to calculate the dihedral angle [closed]

I need to calculate the dihedral angle between C20 and C19, as well as between C19-C18. Can someone help me choosing the right combination? I have used the following one: C18C19C20C23 for the first angle and C20C19C18N7 for the second one. (I am using Maestro software and I am trying to make an energy plot for chi and psi angles). However I am not sure if I should use instead the following combinations: C23C20C19C18 and N7C18C19C20. Does it make any difference? Thank you!

• It makes a difference. Phi and psi are defined for polymers of alpha-amino acids, which this molecules is not. "The phi-angle (φ) - around the N-Cα bond. The psi-angle (ψ) - around the Cα-C bond" (www3.cmbi.umcn.nl/wiki/index.php/Torsion_angle). Jun 28 '20 at 23:40
• it is not about phi and psi angles. this is not a protein. (I mentionned psi and chi angle). this is nelfinavir Jun 29 '20 at 5:12

The picture shows how the dihedral angle $$\psi$$ is defined. You will need to be familiar with vectors to be able to calculate this angle; $$\cos(\psi)=m\cdot n/(|m||n|)$$ where the vectors are shown in the figure, vector $$m$$ is perpendicular to $$a$$ and $$b$$, $$m=a\times b=|a||b|\sin(\gamma)$$ and $$n=b\times c$$.