Change the density threshold for functional evaluation in PySCF

Question

I'm testing some scaling properties of various density functional approximations, which involves evaluating the functionals on model systems with very low density. I am using the funcition dft.libxc.eval_xc in PySCF, yet I found that when the density is lower than some specific thresholds, the function just returns zero instead of evaluating the functional.

I have read the PySCF python source code and found that this might be caused by the default density threshold of libxc, so I guess maybe I can disable that cut-off by modifying the file pyscf/lib/libxc_itrf.so, but that seems kind of complicated for me. So I wonder if there is a more straightforward way to do such thing?

Thanks in advance for the help!

Answer 1

I realize that what I need is only the libxc in PySCF, so I can actually just use libxc rather than PySCF. I installed the python binding of libxc, namely pylibxc, and wrote a function that managed to do what I want:

def eval_xc(xc_code,rho):
    hyb, id_fac = dft.libxc.parse_xc(xc_code)
    inp = {}
    inp["rho"] = rho[0]
    inp["sigma"] = rho[1]**2+rho[2]**2+rho[3]**2
    inp["tau"] = rho[5]
    exc = 0
    for id,fac in id_fac:
        func = pylibxc.LibXCFunctional(id,1)
        func.set_dens_threshold(1E-30)
        ret = func.compute(inp,do_vxc=False)
        temp = ret["zk"].ravel()
        exc = exc + temp*fac
    return exc

Yet I found that when the density is too low, the compute() returns 'nan'. So it seems that the default density threshold is somehow reasonable. Maybe I must a higher precision than float64 to evaluate the functionals on extremely low density systems.