# Molecular structure¶

Modules: gto

## Initializing a molecule¶

There are three ways to define and initialize a molecule. The first is to use the keyword arguments of the Mole.build() method to initialize a molecule:

>>> from pyscf import gto
>>> mol = gto.Mole()
>>> mol.build(
...     atom = '''O 0 0 0; H  0 1 0; H 0 0 1''',
...     basis = 'sto-3g')


The second way is to assign the geometry, basis etc., to the Mole object, followed by calling the build() method:

>>> from pyscf import gto
>>> mol = gto.Mole()
>>> mol.atom = '''O 0 0 0; H  0 1 0; H 0 0 1'''
>>> mol.basis = 'sto-3g'
>>> mol.build()


The third way is to use the shortcut functions pyscf.M() or Mole.M(). These functions pass all the arguments to the build() method:

>>> import pyscf
>>> mol = pyscf.M(
...     atom = '''O 0 0 0; H  0 1 0; H 0 0 1''',
...     basis = 'sto-3g')

>>> from pyscf import gto
>>> mol = gto.M(
...     atom = '''O 0 0 0; H  0 1 0; H 0 0 1''',
...     basis = 'sto-3g')


In any of these, you may have noticed two keywords atom and basis. They are used to hold the molecular geometry and basis sets, which can be defined along with other input options as follows.

### Geometry¶

The molecular geometry can be input in Cartesian format with the default unit being Angstrom (one can specify the unit by setting the attribute unit to either 'Angstrom' or 'Bohr'):

mol = gto.Mole()
mol.atom = '''
O   0. 0. 0.
H   0. 1. 0.
H   0. 0. 1.
'''
mol.unit = 'B' # case insensitive, any string not starts by 'B' or 'AU' is treated as 'Angstrom'


The atoms in the molecule are represented by an element symbol plus three numbers for coordinates. Different atoms should be separated by ; or a line break. In the same atom, , can be used to separate different items. Blank lines or lines started with # will be ignored:

>>> mol = pyscf.M(
... atom = '''
... #O 0 0 0
... H 0 1 0
...
... H 0 0 1
... ''')
>>> mol.natm
2


The input parser also supports the Z-matrix input format:

mol = gto.Mole()
mol.atom = '''
O
H  1  1.2
H  1  1.2  2 105
'''


Similarly, different atoms need to be separated by ; or a line break.

The geometry string is case-insensitive. It also supports to input the nuclear charges of elements:

>>> mol = gto.Mole()
>>> mol.atom = '''8 0. 0. 0.; h 0. 1. 0; H 0. 0. 1.'''


If you need to label an atom to distinguish it from the others, you can prefix or suffix the atom symbol with a number 1234567890 or a special character ~!@#$%^&*()_+.?:<>[]{}| (not , and ;). With this decoration, you can specify different basis sets, masses, or nuclear models on different atoms: >>> mol = gto.Mole() >>> mol.atom = '''8 0 0 0; h:1 0 1 0; H@2 0 0 1''' >>> mol.unit = 'B' >>> mol.basis = {'O': 'sto-3g', 'H': 'cc-pvdz', 'H@2': '6-31G'} >>> mol.build() >>> print(mol._atom) [['O', [0.0, 0.0, 0.0]], ['H:1', [0.0, 1.0, 0.0]], ['H@2', [0.0, 0.0, 1.0]]]  You can also input the geometry in the internal format of Mole.atom: atom = [[atom1, (x, y, z)], [atom2, (x, y, z)], ... [atomN, (x, y, z)]]  This is convenient as you can use Python script to construct the geometry: >>> mol = gto.Mole() >>> mol.atom = [['O',(0, 0, 0)], ['H',(0, 1, 0)], ['H',(0, 0, 1)]] >>> mol.atom.extend([['H', (i, i, i)] for i in range(1,5)])  Besides Python list, tuple and numpy.ndarray are all supported by the internal format: >>> mol.atom = (('O',numpy.zeros(3)), ['H', 0, 1, 0], ['H',[0, 0, 1]])  No matter which format or symbols are used in the input, Mole.build() will convert Mole.atom to the internal format: >>> mol.atom = ''' O 0, 0, 0 ; 1 0.0 1 0 H@2,0 0 1 ''' >>> mol.unit = 'B' >>> mol.build() >>> print(mol._atom) [('O', [0.0, 0.0, 0.0]), ('H', [0.0, 1.0, 0.0]), ('H@2', [0.0, 0.0, 1.0])]  which is stored as the attribute Mole._atom. Once the Mole object is built, the molecular geometry can be accessed through the Mole.atom_coords() function. This function returns a (N,3) array for the coordinates of each atom: >>> print(mol.atom_coords(unit='Bohr')) # unit can be "ANG" or "Bohr" [[0. 0. 0.] [0. 1. 0.] [0. 0. 1.]]  Ghost atoms can also be specified when inputting the geometry. See examples/gto/03-ghost_atom.py for examples. ### Basis set¶ The simplest way to define the basis set is to assign the name of the basis as a string to Mole.basis: mol.basis = 'sto-3g'  This input will apply the specified basis set to all atoms. The name of the basis set in the string is case insensitive. White spaces, dashes and underscores in the name are all ignored. If different basis sets are required for different elements, a Python dict can be used: mol.basis = {'O': 'sto-3g', 'H': '6-31g'}  One can also input custom basis sets with the helper functions. The function gto.basis.parse() can parse a basis string in the NWChem format (https://bse.pnl.gov/bse/portal): mol.basis = {'O': gto.basis.parse(''' C S 71.6168370 0.15432897 13.0450960 0.53532814 3.5305122 0.44463454 C SP 2.9412494 -0.09996723 0.15591627 0.6834831 0.39951283 0.60768372 0.2222899 0.70011547 0.39195739 ''')}  The functions gto.basis.load() can load arbitrary basis sets from the database, even if the basis set does not match the element: mol.basis = {'H': gto.basis.load('sto3g', 'C')}  Both gto.basis.parse() and gto.basis.load() return the basis set in the internal format (see gto_basis). The basis parser also supports ghost atoms: mol.basis = {'GHOST': gto.basis.load('cc-pvdz', 'O'), 'H': 'sto3g'}  More examples of ghost atoms can be found in examples/gto/03-ghost_atom.py. Like the requirements for geometry input, you can use atomic symbols (case-insensitive) or atomic nuclear charges as the keyword of the basis dictionary. Prefix and suffix of numbers and special characters are allowed. If the decorated atomic symbol is appeared in atom but not in basis, the basis parser will remove all decorations and seek the pure atomic symbol in the basis dictionary. In the following example, the 6-31G basis set will be assigned to the atom H1, but the STO-3G basis will be used for the atom H2: mol.atom = ‘8 0 0 0; h1 0 1 0; H2 0 0 1’ mol.basis = {‘O’: ‘sto-3g’, ‘H’: ‘sto3g’, ‘H1’: ‘6-31G’} See examples/gto/04-input_basis.py for more examples. ### ECP¶ Effective core potentials (ECPs) can be specified with the attribute Mole.ecp. Scalar type ECPs are available for all molecular and crystal methods. The built-in scalar ECP datasets include  Keyword Comment bfd cc-pvdz-pp cc-pvtz-pp same to cc-pvdz-pp cc-pvqz-pp same to cc-pvdz-pp cc-pv5z-pp same to cc-pvdz-pp crenbl crenbs def2-svp def2-svpd same to def2-svp def2-tzvp same to def2-svp def2-tzvpd same to def2-svp def2-tzvpp same to def2-svp def2-tzvppd same to def2-svp def2-qzvp same to def2-svp def2-qzvpd same to def2-svp def2-qzvpp same to def2-svp def2-qzvppd same to def2-svp lanl2dz lanl2tz lanl08 sbkjc stuttgart ECP parameters can be specified directly in input script using NWChem format. Examples of ECP input can be found in examples/gto/05-input_ecp.py. Spin-orbit (SO) ECP integrals can be evaluated using PySCF’s integral driver. However, SO-ECPs are not automatically applied to any methods in the current implementation. They need to be added to the core Hamiltonian as shown in the examples examples/gto/20-soc_ecp.py and examples/scf/44-soc_ecp.py. PySCF provides the following SO-ECPs  Keyword Comment crenbl crenbs Note Be careful with the SO-ECP conventions when inputting them directly in the input script. SO-ECP parameters may take different conventions in different packages. More particularly, the treatment of the factor 2/(2l+1). PySCF assumes that this factor has been multiplied into the SOC parameters. See also relevant discussions in Dirac doc and NWChem doc. ### Point group symmetry¶ You can also invoke point group symmetry for molecular calculations by setting the attribute Mole.symmetry to True: >>> mol = pyscf.M( ... atom = 'B 0 0 0; H 0 1 1; H 1 0 1; H 1 1 0', ... symmetry = True ... )  The point group symmetry information is held in the Mole object. The symmetry module (symm) of PySCF can detect arbitrary point groups. The detected point group is saved in Mole.topgroup, and the supported subgroup is saved in Mole.groupname: >>> print(mol.topgroup) C3v >>> print(mol.groupname) Cs  Currently, PySCF supports linear molecular symmetries $$D_{\infty h}$$ (labelled as Dooh in the program) and $$C_{\infty v}$$ (labelled as Coov), the $$D_{2h}$$ group and its subgroups. Sometimes it is necessary to use a lower symmetry instead of the detected symmetry group. The subgroup symmetry can be specified in by Mole.symmetry_subgroup and the program will first detect the highest possible symmetry group and then lower the point group symmetry to the specified subgroup: >>> mol = gto.Mole() >>> mol.atom = 'N 0 0 0; N 0 0 1' >>> mol.symmetry = True >>> mol.symmetry_subgroup = C2 >>> mol.build() >>> print(mol.topgroup) Dooh >>> print(mol.groupname) C2  When a particular symmetry is assigned to Mole.symmetry, the initialization function Mole.build() will test whether the molecule geometry is subject to the required symmetry. If not, initialization will stop and an error message will be issued: >>> mol = gto.Mole() >>> mol.atom = 'O 0 0 0; C 0 0 1' >>> mol.symmetry = 'Dooh' >>> mol.build() RuntimeWarning: Unable to identify input symmetry Dooh. Try symmetry="Coov" with geometry (unit="Bohr") ('O', [0.0, 0.0, -0.809882624813598]) ('C', [0.0, 0.0, 1.0798434997514639])  Note Mole.symmetry_subgroup has no effects when specific symmetry group is assigned to Mole.symmetry. When symmetry is enabled in the Mole object, the point group symmetry information will be used to construct the symmetry adapted orbital basis (see also symm). The symmetry adapted orbitals are held in Mole.symm_orb as a list of 2D arrays. Each element of the list is an AO (atomic orbital) to SO (symmetry-adapted orbital) transformation matrix of an irreducible representation. The name of the irreducible representations are stored in Mole.irrep_name and their internal IDs (see more details in symm) are stored in Mole.irrep_id: >>> mol = gto.Mole() >>> mol.atom = 'O 0 0 0; O 0 0 1.2' >>> mol.spin = 2 >>> mol.symmetry = "D2h" >>> mol.build() >>> for s,i,c in zip(mol.irrep_name, mol.irrep_id, mol.symm_orb): ... print(s, i, c.shape) Ag 0 (10, 3) B2g 2 (10, 1) B3g 3 (10, 1) B1u 5 (10, 3) B2u 6 (10, 1) B3u 7 (10, 1)  These symmetry-adapted orbitals are used as basis functions for the following SCF or post-SCF calculations: >>> mf = scf.RHF(mol) >>> mf.kernel() converged SCF energy = -147.631655286561  and we can check the occupancy of the MOs in each irreducible representation: >>> import numpy >>> from pyscf import symm >>> def myocc(mf): ... mol = mf.mol ... irrep_id = mol.irrep_id ... so = mol.symm_orb ... orbsym = symm.label_orb_symm(mol, irrep_id, so, mf.mo_coeff) ... doccsym = numpy.array(orbsym)[mf.mo_occ==2] ... soccsym = numpy.array(orbsym)[mf.mo_occ==1] ... for ir,irname in enumerate(mol.irrep_name): ... print('%s, double-occ = %d, single-occ = %d' % ... (irname, sum(doccsym==ir), sum(soccsym==ir))) >>> myocc(mf) Ag, double-occ = 3, single-occ = 0 B2g, double-occ = 0, single-occ = 0 B3g, double-occ = 0, single-occ = 1 B1u, double-occ = 0, single-occ = 1 B2u, double-occ = 0, single-occ = 0 B3u, double-occ = 2, single-occ = 0  To label the irreducible representation of given orbitals, symm.label_orb_symm() needs the information of the point group symmetry which is initialized in the Mole object, including the IDs of irreducible representations (Mole.irrep_id) and the symmetry adapted basis Mole.symm_orb. For each irrep_id, Mole.irrep_name gives the associated irrep symbol (A1, B1 …). In the SCF calculation, you can control the symmetry of the wave function by assigning the number of alpha electrons and beta electrons (alpha,beta) for the irreps: >>> mf.irrep_nelec = {'B2g': (1,1), 'B3g': (1,1), 'B2u': (1,0), 'B3u': (1,0)} >>> mf.kernel() converged SCF energy = -146.97333043702 >>> mf.get_irrep_nelec() {'Ag': (3, 3), 'B2g': (1, 1), 'B3g': (1, 1), 'B1u': (2, 2), 'B2u': (1, 0), 'B3u': (1, 0)}  ### Spin and charge¶ Charge and spin multiplicity can be assigned to Mole object: mol.charge = 1 mol.spin = 1  Note Mole.spin is the number of unpaired electrons 2S, i.e. the difference between the number of alpha and beta electrons. These two attributes do not affect any other parameters in the Mole.build initialization function. They can be set or modified after the Mole object is initialized: >>> mol = gto.Mole() >>> mol.atom = 'O 0 0 0; h 0 1 0; h 0 0 1' >>> mol.basis = 'sto-6g' >>> mol.spin = 2 >>> mol.build() >>> print(mol.nelec) (6, 4) >>> mol.spin = 0 >>> print(mol.nelec) (5, 5)  The attribute Mole.charge is the parameter to define the total number of electrons in the system. For custom systems such as the Hubbard lattice model, the total number of electrons needs to be specified directly by setting the attribute Mole.nelectron: >>> mol = gto.Mole() >>> mol.nelectron = 10  ### Other parameters¶ You can assign more information to the molecular object: mol.nucmod = {'O1': 1} # nuclear charge model: 0-point charge, 1-Gaussian distribution mol.mass = {'O1': 18, 'H': 2} # atomic mass  See examples/gto/07-nucmod.py for more examples of nuclear charge models. The Mole class also defines some global parameters. You can control the print level globally with verbose: mol.verbose = 4  The print level can be 0 (quiet, no output) to 9 (very noisy). The most useful messages are printed at level 4 (info) and 5 (debug). You can also specify a place where to write the output messages: mol.output = 'path/to/log.txt'  If this variable is not assigned, messages will be dumped to sys.stdout. The maximum memory usage can be controlled globally: mol.max_memory = 1000 # MB  The default size can also be defined with the shell environment variable PYSCF_MAX_MEMORY. The attributes output and max_memory can also be assigned from command line: $ python input.py -o /path/to/my_log.txt -m 1000


By default, command line has the highest priority, which means the settings in the script will be overwritten by the command line arguments. To make the input parser ignore the command line arguments, you can call the Mole.build() with:

mol.build(0, 0)


The first 0 prevent build() dumping the input file. The second 0 prevent build() parsing the command line arguments.

## Access AO integrals¶

PySCF uses libcint library as the AO integral engine. A simple interface function Mole.intor() is provided to obtain the one- and two-electron AO integrals:

kin = mol.intor('int1e_kin')
vnuc = mol.intor('int1e_nuc')
overlap = mol.intor('int1e_ovlp')
eri = mol.intor('int2e')


For a full list of supported AO integrals, see gto_moleintor.