# 1. set environment
$ source /path/to/oneapi/setvars.sh
$ module load dft4_api

# 2. build vasp
$ cd /path/to/vasp
$ vim makefile.include
# 
# CPP_OPTIONS += -DDFTD4
# DFTD4_ROOT  ?= /path/to/dftd4/
# LLIBS       += -L$(DFTD4_ROOT)/lib64 -ldftd4
# INCS        += -I$(DFTD4_ROOT)/include -I$(DFTD4_ROOT)/include/dftd4/IntelLLVM-2023.1.0
# 
# LLIBS and INCS are the result of:
# pkg-config --libs dftd4_api  and  pkg-config --cflag dftd4_api 
# separately. 

$ make veryclean
$ make DEPS=1 -j
#
# the below is the make output
#
mpiifort -qmkl=sequential  # <-- here errors occurs when linking targets
   -o vasp c2f_interface.o ... subdftd3.o subdftd4.o ... main.o 
   -Llib -ldmy -Lparser -lparser -lstdc++ 
    -L/path/to/oneapi/mkl/2023.1.0/lib/intel64 
   -lmkl_scalapack_lp64 -lmkl_blacs_intelmpi_lp64 
   -L/path/to/hdf5/latest/lib -lhdf5_fortran 
   -L/path/to/dftd4/lib64 -ldftd4
# error output: 
/path/to/dftd4_api/lib64/libdftd4.a(compat.f90.o): In function `dftd4_compat_mp_d4_calculation_':
/path/to/dftd4_install/src/dftd4/compat.f90:111: undefined reference to `mctc_io_structure_mp_new_structure_num_'
/path/to/dftd4_install//3.6.0/src/dftd4/compat.f90:125: undefined reference to `mctc_io_math_mp_matinv_3x3_'
#
... # (Here I truncate the error output because too many 'undefined reference to')
#
/path/to/dftd4_api/lib64/libdftd4.a(charge.f90.o): In function `dftd4_charge_mp_get_charges_':
/path/to/dftd4_install/src/dftd4/charge.f90:57: undefined reference to `multicharge_param_mp_new_eeq2019_model_'
/path/to/dftd4_install//src/dftd4/charge.f90:58: undefined reference to `multicharge_cutoff_mp_get_lattice_points_cutoff_'
/path/to/dftd4_install//src/dftd4/charge.f90:65: undefined reference to /path/to/dftd4_install//src/dftd4/charge.f90:66: undefined reference to `multicharge_ncoord_mp_get_coordination_number_'
/path/to/dftd4_install//src/dftd4/charge.f90:68: undefined reference to `multicharge_model_mp_solve_'
/path/to/dftd4_install//lib64/libdftd4.a(charge.f90.o):(.data+0x10): undefined reference to `multicharge_model_mp_solve_'
make[2]: *** [makefile:132: vasp] Error 1

$ cd /path/to/dftd4_install

$ vim ./src/dftd4/charge.f90
# 
#  18 module dftd4_charge
# 
#  21    use multicharge, only : mchrg_model_type, new_eeq2019_model, &  
# 
#  27    public :: get_charges
# 
#  34 subroutine get_charges(mol, qvec, dqdr, dqdL)
#  35    !DEC$ ATTRIBUTES DLLEXPORT :: get_charges
# 
#  70 end subroutine get_charges
# 
#  73 end module dftd4_charge

$ vim ./subprojects/multicharge/src/multicharge/param.f90
#
# 16 module multicharge_param
#
# 25    public :: new_eeq2019_model
# 
# 29 subroutine new_eeq2019_model(mol, model)
# 
# 44 end subroutine new_eeq2019_model
#
# 46 end module multicharge_param