Incorrect sign in Debye-Hückel electron-nucleus screening

[sadasivanmj]

I found what appears to be a sign error in the electron-nucleus
Debye-Hückel screening implementation in orbital.c.

For debye_mode=1, the current implementation constructs

ZPS = Z (exp(-r/lambda_D) - 1)

which gives

V_en = (-Z + ZPS)/r
     = Z(exp(-r/lambda_D) - 2)/r,

rather than the expected Yukawa potential

V_en = -Z exp(-r/lambda_D)/r.

Changing this to

ZPS = Z (1 - exp(-r/lambda_D))

and making the corresponding derivative changes gives

V_en = -Z exp(-r/lambda_D)/r.

I have verified the corrected potential directly against the
analytic Yukawa expression and tested the resulting self-consistent
FAC calculations for He-like C4+, Ne8+, Ar16+, and Kr34+.

I have a minimal patch available and would be happy to submit it
as a pull request if this interpretation of debye_mode=1 is correct.

[mfgu]

this sounds right. I will push the fix, no need to do a pull request. Thanks.