CCL:G: Unrestricted vs. restricted DFT for closed-shell metal complex

["Andrew Scott Rosen" <rosen _ u.northwestern.edu>]

 Sent to CCL by: "Andrew Scott Rosen" [rosen]^[u.northwestern.edu]
 Hello,
 I have a question regarding the choice of unrestricted versus restricted
 calculations in DFT. Let's say that I am dealing with a Cu(I)
 organometallic complex that I anticipate would have a spin multiplicity of
 1 due to the assumed [Ar]3d10 electron configuration of Cu(I) in this
 complex. Under this assumption, the system is not open-shell. As such, it
 seems that a restricted calculation would be most appropriate.
 Would there be any benefit in running an unrestricted calculation (still as
 a singlet) in this case? Additionally, what would an unrestricted
 calculation on a system that's a singlet even mean, especially if I don't
 use specific keywords (e.g. in Gaussian) to try and obtain a broken-
 symmetry state?
 To provide context, I ask because I start with a Cu(II) complex that
 reduces to a Cu(I) complex. I do an unrestricted calculation when modeling
 the Cu(II) complex, and I want to compute the energy of the resulting Cu(I)
 complex for comparison.
 Thank you greatly for any input.
 Best regards,
 Andrew