An empty atom is defined in the STRU file when an element name contains the "empty" suffix, such as "H_empty" in the following example: calculating the molecular formation energy of H$_2$O by using the BSSE (Basis Set Superposition Errors) method.
ntype 2
ATOMIC_SPECIES
H 1.008 H_ONCV_PBE-1.0.upf
O 15.9994 O_ONCV_PBE-1.0.upf
NUMERICAL_ORBITAL
H_gga_8au_60Ry_2s1p.orb
O_gga_6au_60Ry_2s2p1d.orb
LATTICE_CONSTANT
1.889725989
LATTICE_VECTORS
20 0 0
0 20 0
0 0 20
ATOMIC_POSITIONS
Cartesian # Cartesian(Unit is LATTICE_CONSTANT)
H
0.5
2
0.9584 0.0000 0.0000 0 0 0
-0.2392 0.9281 0.0000 0 0 0
O
0.5
1
0.0000 0.0000 0.0000 0 0 0
ntype 2
ATOMIC_SPECIES
H_empty 1.008 H_ONCV_PBE-1.0.upf
O 15.9994 O_ONCV_PBE-1.0.upf
NUMERICAL_ORBITAL
H_gga_8au_60Ry_2s1p.orb
O_gga_6au_60Ry_2s2p1d.orb
LATTICE_CONSTANT
1.889725989
LATTICE_VECTORS
20 0 0
0 20 0
0 0 20
ATOMIC_POSITIONS
Cartesian # Cartesian(Unit is LATTICE_CONSTANT)
H_empty
0.5
2
0.9584 0.0000 0.0000 0 0 0
-0.2392 0.9281 0.0000 0 0 0
O
0.5
1
0.0000 0.0000 0.0000 0 0 0
ntype 3
ATOMIC_SPECIES
H 1.008 H_ONCV_PBE-1.0.upf
H_empty_2 1.008 H_ONCV_PBE-1.0.upf
O_empty 15.9994 O_ONCV_PBE-1.0.upf
NUMERICAL_ORBITAL
H_gga_8au_60Ry_2s1p.orb
H_gga_8au_60Ry_2s1p.orb
O_gga_6au_60Ry_2s2p1d.orb
LATTICE_CONSTANT
1.889725989
LATTICE_VECTORS
20 0 0
0 20 0
0 0 20
ATOMIC_POSITIONS
Cartesian # Cartesian(Unit is LATTICE_CONSTANT)
H
0.5
1
0.9584 0.0000 0.0000 0 0 0
H_empty_2
0.5
1
-0.2392 0.9281 0.0000 0 0 0
O_empty
0.5
1
0.0000 0.0000 0.0000 0 0 0
ntype 3
ATOMIC_SPECIES
H_empty_1 1.008 H_ONCV_PBE-1.0.upf
H 1.008 H_ONCV_PBE-1.0.upf
O_empty 15.9994 O_ONCV_PBE-1.0.upf
NUMERICAL_ORBITAL
H_gga_8au_60Ry_2s1p.orb
H_gga_8au_60Ry_2s1p.orb
O_gga_6au_60Ry_2s2p1d.orb
LATTICE_CONSTANT
1.889725989
LATTICE_VECTORS
20 0 0
0 20 0
0 0 20
ATOMIC_POSITIONS
Cartesian # Cartesian(Unit is LATTICE_CONSTANT)
H_empty_1
0.5
1
0.9584 0.0000 0.0000 0 0 0
H
0.5
1
-0.2392 0.9281 0.0000 0 0 0
O_empty
0.5
1
0.0000 0.0000 0.0000 0 0 0