CAM-B3LYP calculation via libxc has no exx

[jeanwsr]

Describe the bug

This is a band job generated by AsterFire Go platform (ABACUS v3.10.1):

INPUT_PARAMETERS

...

# Electronic structure
basis_type          lcao
ks_solver           cusolver
dft_functional      HYB_GGA_XC_CAM_B3LYP
smearing_method     gauss
smearing_sigma      0.004
mixing_type         broyden
mixing_beta         0.4
mixing_ndim         20
mixing_gg0          1.0
gamma_only          0
scf_nmax            100
scf_thr             1e-07

...

In running_scf.log I found

 LCAO ALGORITHM --------------- ION=   1  ELEC=  14--------------------------------

 Density error is 6.11145223884e-08
----------------------------------------------------------
     Energy           Rydberg                 eV          
----------------------------------------------------------
 E_KohnSham     -21.1420813815       -287.6527743683      
 E_KS(sigma->0) -21.1420813815       -287.6527743683      
 E_Harris       -21.1420826434       -287.6527915366      
 E_band         -4.0734737497        -55.4224536500       
 E_one_elec     -157.0637584030      -2136.9620635763     
 E_Hartree      79.6573363944        1083.7936624667      
 E_xc           -5.6240622546        -76.5192925694       
 E_Ewald        61.8884028817        842.0349193107       
 E_entropy(-TS) -0.0000000000        -0.0000000000        
 E_descf        0.0000000000         0.0000000000         
 E_exx          0.0000000000         0.0000000000         
 E_Fermi        0.2450171022         3.3336286970         
----------------------------------------------------------

which seems incorrect since CAM-B3LYP is a RSH functional.

Expected behavior

CAM-B3LYP should have E_exx.

To Reproduce

No response

Environment

No response

Additional Context

No response

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[AsTonyshment]

Hi, to the best of my knowledge and based on my previous experience, ABACUS does not currently support arbitrary hybrid functionals simply by specifying their Libxc names. Hybrid functionals need to be explicitly supported by ABACUS itself; see https://abacus.deepmodeling.com/en/latest/advanced/input_files/input-main.html#dft-functional.

About a year and a half ago, I investigated this and implemented the BHLYP hybrid functional in my own fork. At that time, simply specifying a hybrid functional through Libxc was not sufficient to activate the corresponding exact-exchange calculation, and I believe the same limitation applies here.

[AsTonyshment]

@jeanwsr If CAM-B3LYP is urgently needed, we can consider adding explicit support for this functional. In the longer term, supporting arbitrary hybrid functionals through Libxc identifiers would certainly be valuable. However, this would likely require a substantial refactoring of the current XC module, so a general solution may not be available in the near future. 😭

[jeanwsr]

Thanks for the response. I think it's not hard to implement support for arbitrary Hyb/RSH libxc functional. For the type one can query XC_FLAGS_HYB_CAM, XC_FAMILY_HYB_*. For the coeff one can use xc_hyb_cam_coef. Although I know nothing about abacus development but these things are quite common in my developing experience of other programs that uses libxc.

[jeanwsr]

@jeanwsr If CAM-B3LYP is urgently needed, we can consider adding explicit support for this functional. In the longer term, supporting arbitrary hybrid functionals through Libxc identifiers would certainly be valuable. However, this would likely require a substantial refactoring of the current XC module, so a general solution may not be available in the near future. 😭

Thanks, that's not urgent. We can leave this issue open as a feature request.