a bug in hfx_energy_potential.F for multiple HF sections

[Hanning Chen]

Dear CP2K developers,

  I would like to report a bug when multiple Hartree-Fock (HF) sections are
used.  It is well known that a HF coulomb potential (1/r) can be decomposed
into a short-range potential (erfc(wr)/r) and a long-range potential
(erf(wr)/r). Therefore, the two attached input files named "HF.inp" and
"RS.inp" should afford the same energy. Unfortunately, using the latest
CP2K 6.0 code (svn: 18109), they yield distinct values: HF -37.59 vs RS
-41.92. I think an easy fix is to modify Line 2945 of
hfx_energy_potential.F

        from         energy%ex = 0.5_dp*(ene_x_aa + ene_x_bb) + energy%ex

        to             energy%ex = 0.5_dp*(ene_x_aa + ene_x_bb)

After the easy fix, the two input files achieve the same energy of -37.59.

 Please correct me if there exists any misunderstanding.

  Thanks.

Hanning
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