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PR-6 - feat: add C shell loop binding for 2-electron ERI #254

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PR-6 - feat: add C shell loop binding for 2-electron ERI #254

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d2a25e5
feat: add C shell loop for 2-electron ERI
Jun 24, 2026
5ba1d00
refactor: rename macros to DEFINE_INT1E_ARRAY_FN and DEFINE_INT1E_SHE…
Jun 26, 2026
8bd6884
fix: fill ERI in physicist notation in C
Jun 26, 2026
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24 changes: 23 additions & 1 deletion gbasis/integrals/libcint.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1309,7 +1309,29 @@ def octupole(self):
)
return out


def electron_repulsion(self):
r"""
Compute the electron repulsion integrals.

The two-electron repulsion integral between basis functions
Comment on lines +1312 to +1316
:math:`\phi_i`, :math:`\phi_j`, :math:`\phi_k`, and :math:`\phi_l`
is defined as:

.. math::
g_{ijkl} = \langle \phi_i \phi_j | \frac{1}{r_{12}} | \phi_k \phi_l \rangle

Returns
-------
out : np.ndarray(Nbasis, Nbasis, Nbasis, Nbasis, dtype=float)
Electron repulsion integral array.

"""
out = np.zeros((self.nbfn, self.nbfn, self.nbfn, self.nbfn), dtype=c_double)
libcint_bindings.eri_shellloop(
out, self.natm, self.atm, self.nbas,
self.bas, self.env, self._offs, self.nbfn
)
return out
Comment on lines +1329 to +1334

def electron_repulsion_integral(self, notation="physicist", transform=None):
r"""
Expand Down
119 changes: 98 additions & 21 deletions gbasis/integrals/src/libcint_wrap.c
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Original file line number Diff line number Diff line change
Expand Up @@ -68,13 +68,13 @@ extern int int1e_rrr_sph(double *out, int *dims, int *shls, int *atm, int natm,
extern int int2e_sph(double *out, int *dims, int *shls, int *atm, int natm, int *bas, int nbas, double *env, void *opt, double *cache);

/*
* DEFINE_INTEGRAL_INT1e(func_name, libcint_func)
* DEFINE_INT1E_ARRAY_FN(func_name, libcint_func)
* Generates a Python/C API wrapper for a 1-electron libcint integral.
* Accepts NumPy arrays directly — uses PyArray_GETPTR1 (NumPy C-API).
*/

/* Macro for 1-electron wrappers */
#define DEFINE_INTEGRAL_INT1e(func_name, libcint_func) \
#define DEFINE_INT1E_ARRAY_FN(func_name, libcint_func) \
static PyObject * \
func_name(PyObject *self, PyObject *args) \
{ \
Expand All @@ -99,15 +99,15 @@ func_name(PyObject *self, PyObject *args) \
return PyLong_FromLong(result); \
}

DEFINE_INTEGRAL_INT1e(overlap_sph, int1e_ovlp_sph)
DEFINE_INTEGRAL_INT1e(kinetic_sph, int1e_kin_sph)
DEFINE_INTEGRAL_INT1e(nuclear_sph, int1e_nuc_sph)
DEFINE_INTEGRAL_INT1e(momentum_sph, int1e_ipovlp_sph)
DEFINE_INTEGRAL_INT1e(angular_momentum_sph, int1e_cg_irxp_sph)
DEFINE_INTEGRAL_INT1e(rinv_sph, int1e_rinv_sph)
DEFINE_INTEGRAL_INT1e(dipole_sph, int1e_r_sph)
DEFINE_INTEGRAL_INT1e(quadrupole_sph, int1e_rr_sph)
DEFINE_INTEGRAL_INT1e(octupole_sph, int1e_rrr_sph)
DEFINE_INT1E_ARRAY_FN(overlap_sph, int1e_ovlp_sph)
DEFINE_INT1E_ARRAY_FN(kinetic_sph, int1e_kin_sph)
DEFINE_INT1E_ARRAY_FN(nuclear_sph, int1e_nuc_sph)
DEFINE_INT1E_ARRAY_FN(momentum_sph, int1e_ipovlp_sph)
DEFINE_INT1E_ARRAY_FN(angular_momentum_sph, int1e_cg_irxp_sph)
DEFINE_INT1E_ARRAY_FN(rinv_sph, int1e_rinv_sph)
DEFINE_INT1E_ARRAY_FN(dipole_sph, int1e_r_sph)
DEFINE_INT1E_ARRAY_FN(quadrupole_sph, int1e_rr_sph)
DEFINE_INT1E_ARRAY_FN(octupole_sph, int1e_rrr_sph)

/* 2-electron wrapper */

Expand Down Expand Up @@ -142,13 +142,13 @@ electron_repulsion_sph(PyObject *self, PyObject *args)
}

/*
* DEFINE_SHELLLOOP_INT1e(func_name, libcint_func)
* DEFINE_INT1E_SHELLLOOP_FN(func_name, libcint_func)
* Generates a C shell-loop wrapper for a 1-electron libcint integral that
* returns the FULL integral array over all shells (not just one shell pair).
* Loops over shells I, J; calls libcint_func per shell pair; fills the
* symmetric output matrix using PyArray_GETPTR (NumPy C-API).
*/
#define DEFINE_SHELLLOOP_INT1e(func_name, libcint_func) \
#define DEFINE_INT1E_SHELLLOOP_FN(func_name, libcint_func) \
static PyObject * \
func_name(PyObject *self, PyObject *args) \
{ \
Expand Down Expand Up @@ -196,17 +196,93 @@ func_name(PyObject *self, PyObject *args)
}

/* Generate shell-loop wrappers for all 1-electron integrals using the macro */
DEFINE_SHELLLOOP_INT1e(overlap_integral_shellloop, int1e_ovlp_sph)
DEFINE_SHELLLOOP_INT1e(kinetic_integral_shellloop, int1e_kin_sph)
DEFINE_SHELLLOOP_INT1e(nuclear_integral_shellloop, int1e_nuc_sph)
DEFINE_SHELLLOOP_INT1e(momentum_integral_shellloop, int1e_ipovlp_sph)
DEFINE_SHELLLOOP_INT1e(rinv_integral_shellloop, int1e_rinv_sph)
DEFINE_SHELLLOOP_INT1e(dipole_integral_shellloop, int1e_r_sph)
DEFINE_SHELLLOOP_INT1e(quadrupole_integral_shellloop, int1e_rr_sph)
DEFINE_SHELLLOOP_INT1e(octupole_integral_shellloop, int1e_rrr_sph)
DEFINE_INT1E_SHELLLOOP_FN(overlap_integral_shellloop, int1e_ovlp_sph)
DEFINE_INT1E_SHELLLOOP_FN(kinetic_integral_shellloop, int1e_kin_sph)
DEFINE_INT1E_SHELLLOOP_FN(nuclear_integral_shellloop, int1e_nuc_sph)
DEFINE_INT1E_SHELLLOOP_FN(momentum_integral_shellloop, int1e_ipovlp_sph)
DEFINE_INT1E_SHELLLOOP_FN(rinv_integral_shellloop, int1e_rinv_sph)
DEFINE_INT1E_SHELLLOOP_FN(dipole_integral_shellloop, int1e_r_sph)
DEFINE_INT1E_SHELLLOOP_FN(quadrupole_integral_shellloop, int1e_rr_sph)
DEFINE_INT1E_SHELLLOOP_FN(octupole_integral_shellloop, int1e_rrr_sph)

/* eri_shellloop — shell-by-shell loop in C for 2-electron ERI (4 shells: I,J,K,L) */
static PyObject *
eri_shellloop(PyObject *self, PyObject *args)
{
PyArrayObject *out_arr, *atm_arr, *bas_arr, *env_arr, *offs_arr;
int natm, nbas, nbfn;

if (!PyArg_ParseTuple(args, "O!iO!iO!O!O!i",
&PyArray_Type, &out_arr,
&natm,
&PyArray_Type, &atm_arr,
&nbas,
&PyArray_Type, &bas_arr,
&PyArray_Type, &env_arr,
&PyArray_Type, &offs_arr,
&nbfn))
return NULL;

double *out = (double *)PyArray_DATA(out_arr);
int *atm = (int *) PyArray_GETPTR2(atm_arr, 0, 0);
int *bas = (int *) PyArray_GETPTR2(bas_arr, 0, 0);
double *env = (double *)PyArray_GETPTR1(env_arr, 0);
int *offs = (int *) PyArray_GETPTR1(offs_arr, 0);

int shls[4];
double buf[100000] = {0};

int ipos = 0;
Comment on lines +232 to +235
for (int ishl = 0; ishl < nbas; ishl++) {
shls[0] = ishl;
int p_off = offs[ishl];
int jpos = 0;
for (int jshl = 0; jshl <= ishl; jshl++) {
int ij = ((ishl + 1) * ishl) / 2 + jshl;
shls[1] = jshl;
int q_off = offs[jshl];
int kpos = 0;
for (int kshl = 0; kshl < nbas; kshl++) {
shls[2] = kshl;
int r_off = offs[kshl];
int lpos = 0;
for (int lshl = 0; lshl <= kshl; lshl++) {
int kl = ((kshl + 1) * kshl) / 2 + lshl;
if (ij < kl) {
lpos += offs[lshl];
continue;
}
shls[3] = lshl;
int s_off = offs[lshl];
int2e_sph(buf, NULL, shls, atm, natm, bas, nbas, env, NULL, NULL);
for (int p = 0; p < p_off; p++) {
for (int q = 0; q < q_off; q++) {
for (int r = 0; r < r_off; r++) {
for (int s = 0; s < s_off; s++) {
double val = buf[p + p_off*(q + q_off*(r + r_off*s))];
int i = ipos+p, j = jpos+q, k = kpos+r, l = lpos+s;
out[i*nbfn*nbfn*nbfn + k*nbfn*nbfn + j*nbfn + l] = val;
out[i*nbfn*nbfn*nbfn + l*nbfn*nbfn + j*nbfn + k] = val;
out[j*nbfn*nbfn*nbfn + k*nbfn*nbfn + i*nbfn + l] = val;
out[j*nbfn*nbfn*nbfn + l*nbfn*nbfn + i*nbfn + k] = val;
out[k*nbfn*nbfn*nbfn + i*nbfn*nbfn + l*nbfn + j] = val;
out[k*nbfn*nbfn*nbfn + j*nbfn*nbfn + l*nbfn + i] = val;
out[l*nbfn*nbfn*nbfn + i*nbfn*nbfn + k*nbfn + j] = val;
out[l*nbfn*nbfn*nbfn + j*nbfn*nbfn + k*nbfn + i] = val; }
}
}
}
memset(buf, 0, sizeof(buf));
lpos += s_off;
}
kpos += r_off;
}
jpos += q_off;
}
ipos += p_off;
}
Py_RETURN_NONE;
}


static PyMethodDef LibcintMethods[] = {
Expand All @@ -228,6 +304,7 @@ static PyMethodDef LibcintMethods[] = {
{"dipole_integral_shellloop", dipole_integral_shellloop, METH_VARARGS, "Dipole integral shell loop in C"},
{"quadrupole_integral_shellloop", quadrupole_integral_shellloop, METH_VARARGS, "Quadrupole integral shell loop in C"},
{"octupole_integral_shellloop", octupole_integral_shellloop, METH_VARARGS, "Octupole integral shell loop in C"},
{"eri_shellloop", eri_shellloop, METH_VARARGS, "ERI 2-electron shell loop in C"},
{NULL, NULL, 0, NULL}
};

Expand Down
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