Question: meta-GGA implementation in ABACUS

[AsTonyshment]

Details

I am reading module_hamilt_general/module_xc/xc_functional_vxc.cpp, and I am confused about meta-GGA implementation in ABACUS.

According to libxc manual https://libxc.gitlab.io/manual/libxc-5.1.x/, xc_mgga_exc_vxc is defined as follow:

void xc_mgga_exc_vxc(const xc_func_type *p, int np, double *rho, double *sigma, double *lapl, double *tau, 
     double * exc, double *vrho, double *vsigma, double *vlapl, double *vtau)

However in the ABACUS code, it seems that lapl(i.e. $$\nabla^2\rho$$) is not used:

xc_mgga_exc_vxc(&func, nrxx, rho.data(), sigma.data(), sigma.data(),
                kin_r.data(), exc.data(), vrho.data(), vsigma.data(), vlapl.data(), vtau.data());

Instead sigma(i.e. $$\nabla\rho\cdot \nabla\rho$$) is passed. Is this correct?

Have you read FAQ on the online manual http://abacus.deepmodeling.com/en/latest/community/faq.html

• Yes, I have read the FAQ part on online manual.

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[WHUweiqingzhou]

@jinzx10 could you have a look?

[jinzx10]

@jinzx10 could you have a look?

I think this should be reviewed by a density functional expert.

[jeanwsr]

Laplacian is not used by most MGGAs. You can query libxc to know whether lapl is needed for certain functional like
https://github.com/pyscf/pyscf/blob/b519c3dd4935e97dac2d646fc75dc5739911559e/pyscf/lib/dft/libxc_itrf.c#L711

[AsTonyshment]

I see. So should we implement checks to ensure that users do not apply mGGA functionals with the Laplacian? Alternatively, we could add a function to compute $\nabla^2\rho$. In either case, we should ensure that this does not lead to UNDEFINED BEHAVIOUR in the underlying code.

[jeanwsr]

Yeah, in principle there should be a check. (the lapl-needed mGGAs are very rare, though)