second derivative of Hartree-Fock exact exchange

[NUCP2K]

Dear CP2K folks,

  I am wondering if the second derivative of Hartree-Fock exact exchange 
with respect to electron density has been implemented?

  The hfx_derivatives.f provides a public subroutine called 
"derivatives_four_center". But not sure if it is the first or second 
derivative.

  Thanks.

Hanning
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