This is the ‘orbital' module, which is used to compute
"Two-Center Integrals (TCI)" by using the Numerical Atomic Orbitals (NAO).

Explain TCI:
TCI is defined as <phi_i | phi_j> or <dphi_i/dR | phi_j>, where
i and j are indexes of numerical atomic orbitalas.
Furthermore, the module is expected to compute formulas such as 
<phi_i | r | phi_j>

Explain NAO:
Numerical atomic orbitals can be divided into different categories,
including 
1) numerical atomic orbitals for expanding wave functions of systems,
which are named 'phi'
2) non-local projectors, which are named 'beta'
3) descriptors for generating labels of electronic structure information,
which are named 'descriptors'

Let me briefly describe the files contained in this 'ORB' module.
--------------------------------------------------------------------
ABACUS-ORB.x: the .exe file of the module
main.cpp: main program file
ORB_atomic.cpp: information for all of the atomic orbitals
ORB_atomic.h
ORB_atomic_lm.cpp: store each atomic orbitals
ORB_atomic_lm.h
ORB_gaunt_table.cpp: compute Gaunt tables
ORB_gaunt_table.h
ORB_nonlocal.cpp: information for all of the non-local projectors
ORB_nonlocal.h
ORB_nonlocal_lm.cpp: store each non-local projector
ORB_nonlocal_lm.h
ORB_read.cpp: read-in basis sets/non-local projectors/descriptors
ORB_read.h
--------------------------------------------------------------------

There are some other files in the 'OBJS_BASE' group (mainly math functions), 
which we will give detaied explanations in near future.

You may see codes with __NORMAL defined. Those codes compiled with __NORMAL
are used in this ORB module, while those codes compiled without __NORMAL are
used in the ABACUS code. We will keep on optimizing those codes in near fugure.

-- Mohan Chen 2021-05-21 mohanchen@pku.edu.cn