Add general integer exponents

docs/src/references/changelog.rst

@@ -32,6 +32,8 @@ Added
 Fixed
 #####
 
+* Refactor the ``InversePowerLawPotential`` class to restrict the exponent to integer
+  values
 * Ensured consistency of ``dtype`` and ``device`` in the ``Potential`` and
   ``Calculator`` classses
 * Fixed consistency of ``dtype`` and ``device`` in the ``SplinePotential`` class

examples/8-combined-potential.py

@@ -65,8 +65,8 @@
 # evaluation, and so one has to set it also for the combined potential, even if it is
 # not used explicitly in the evaluation of the combination.
 
-pot_1 = InversePowerLawPotential(exponent=1.0, smearing=smearing)
-pot_2 = InversePowerLawPotential(exponent=2.0, smearing=smearing)
+pot_1 = InversePowerLawPotential(exponent=1, smearing=smearing)
+pot_2 = InversePowerLawPotential(exponent=2, smearing=smearing)
 
 potential = CombinedPotential(potentials=[pot_1, pot_2], smearing=smearing)
 

src/torchpme/calculators/ewald.py

@@ -138,6 +138,5 @@ def _compute_kspace(
         charge_tot = torch.sum(charges, dim=0)
         prefac = self.potential.background_correction()
         energy -= 2 * prefac * charge_tot * ivolume
-
         # Compensate for double counting of pairs (i,j) and (j,i)
         return energy / 2

src/torchpme/lib/__init__.py

@@ -4,6 +4,7 @@
     generate_kvectors_for_mesh,
     get_ns_mesh,
 )
+from .math import CustomExp1, gamma, gammaincc_over_powerlaw, torch_exp1
 from .mesh_interpolator import MeshInterpolator
 
 __all__ = [
@@ -16,4 +17,8 @@
     "generate_kvectors_for_ewald",
     "generate_kvectors_for_mesh",
     "get_ns_mesh",
+    "gamma",
+    "CustomExp1",
+    "gammaincc_over_powerlaw",
+    "torch_exp1",
 ]

src/torchpme/lib/math.py

@@ -0,0 +1,56 @@
+import torch
+from scipy.special import exp1
+from torch.special import gammaln
+
+
+def gamma(x: torch.Tensor) -> torch.Tensor:
+    """
+    (Complete) Gamma function.
+
+    pytorch has not implemented the commonly used (complete) Gamma function. We define
+    it in a custom way to make autograd work as in
+    https://discuss.pytorch.org/t/is-there-a-gamma-function-in-pytorch/17122
+    """
+    return torch.exp(gammaln(x))
+
+
+class CustomExp1(torch.autograd.Function):
+    """Custom exponential integral function Exp1(x) to have an autograd-compatible version."""
+
+    @staticmethod
+    def forward(ctx, input):
+        ctx.save_for_backward(input)
+        input_numpy = input.cpu().numpy() if not input.is_cpu else input.numpy()
+        return torch.tensor(exp1(input_numpy), device=input.device, dtype=input.dtype)
+
+    @staticmethod
+    def backward(ctx, grad_output):
+        (input,) = ctx.saved_tensors
+        return -grad_output * torch.exp(-input) / input
+
+
+def torch_exp1(input):
+    """Wrapper for the custom exponential integral function."""
+    return CustomExp1.apply(input)
+
+
+def gammaincc_over_powerlaw(exponent: torch.Tensor, z: torch.Tensor) -> torch.Tensor:
+    """Function to compute the regularized incomplete gamma function complement for integer exponents."""
+    if exponent == 1:
+        return torch.exp(-z) / z
+    if exponent == 2:
+        return torch.sqrt(torch.pi / z) * torch.erfc(torch.sqrt(z))
+    if exponent == 3:
+        return torch_exp1(z)
+    if exponent == 4:
+        return 2 * (
+            torch.exp(-z) - torch.sqrt(torch.pi * z) * torch.erfc(torch.sqrt(z))
+        )
+    if exponent == 5:
+        return torch.exp(-z) - z * torch_exp1(z)
+    if exponent == 6:
+        return (
+            (2 - 4 * z) * torch.exp(-z)
+            + 4 * torch.sqrt(torch.pi * z**3) * torch.erfc(torch.sqrt(z))
+        ) / 3
+    raise ValueError(f"Unsupported exponent: {exponent}")

src/torchpme/potentials/inversepowerlaw.py

@@ -1,20 +1,11 @@
 from typing import Optional
 
 import torch
-from torch.special import gammainc, gammaincc, gammaln
+from torch.special import gammainc
 
-from .potential import Potential
-
-
-def gamma(x: torch.Tensor) -> torch.Tensor:
-    """
-    (Complete) Gamma function.
+from torchpme.lib import gamma, gammaincc_over_powerlaw
 
-    pytorch has not implemented the commonly used (complete) Gamma function. We define
-    it in a custom way to make autograd work as in
-    https://discuss.pytorch.org/t/is-there-a-gamma-function-in-pytorch/17122
-    """
-    return torch.exp(gammaln(x))
+from .potential import Potential
 
 
 class InversePowerLawPotential(Potential):
@@ -46,16 +37,16 @@ class InversePowerLawPotential(Potential):
 
     def __init__(
         self,
-        exponent: float,
+        exponent: int,
         smearing: Optional[float] = None,
         exclusion_radius: Optional[float] = None,
         dtype: Optional[torch.dtype] = None,
         device: Optional[torch.device] = None,
     ):
         super().__init__(smearing, exclusion_radius, dtype, device)
 
-        if exponent <= 0 or exponent > 3:
-            raise ValueError(f"`exponent` p={exponent} has to satisfy 0 < p <= 3")
+        # function call to check the validity of the exponent
+        gammaincc_over_powerlaw(exponent, torch.tensor(1.0, dtype=dtype, device=device))
         self.register_buffer(
             "exponent", torch.tensor(exponent, dtype=self.dtype, device=self.device)
         )
@@ -130,9 +121,7 @@ def lr_from_k_sq(self, k_sq: torch.Tensor) -> torch.Tensor:
         # for consistency reasons.
         masked = torch.where(x == 0, 1.0, x)  # avoid NaNs in backwards, see Coulomb
         return torch.where(
-            k_sq == 0,
-            0.0,
-            prefac * gammaincc(peff, masked) / masked**peff * gamma(peff),
+            k_sq == 0, 0.0, prefac * gammaincc_over_powerlaw(exponent, masked)
         )
 
     def self_contribution(self) -> torch.Tensor:
@@ -145,7 +134,11 @@ def self_contribution(self) -> torch.Tensor:
         return 1 / gamma(phalf + 1) / (2 * self.smearing**2) ** phalf
 
     def background_correction(self) -> torch.Tensor:
-        # "charge neutrality" correction for 1/r^p potential
+        # "charge neutrality" correction for 1/r^p potential diverges for exponent p = 3
+        # and is not needed for p > 3 , so we set it to zero (see in
+        # https://doi.org/10.48550/arXiv.2412.03281 SI section)
+        if self.exponent >= 3:
+            return torch.tensor(0.0, dtype=self.dtype, device=self.device)
         if self.smearing is None:
             raise ValueError(
                 "Cannot compute background correction without specifying `smearing`."

tests/calculators/test_values_ewald.py

@@ -100,7 +100,7 @@ def test_madelung(crystal_name, scaling_factor, calc_name):
         lr_wavelength = 0.5 * smearing
         calc = EwaldCalculator(
             InversePowerLawPotential(
-                exponent=1.0,
+                exponent=1,
                 smearing=smearing,
             ),
             lr_wavelength=lr_wavelength,
@@ -111,7 +111,7 @@ def test_madelung(crystal_name, scaling_factor, calc_name):
         smearing = sr_cutoff / 5.0
         calc = PMECalculator(
             InversePowerLawPotential(
-                exponent=1.0,
+                exponent=1,
                 smearing=smearing,
             ),
             mesh_spacing=smearing / 8,
@@ -198,7 +198,7 @@ def test_wigner(crystal_name, scaling_factor):
         # Compute potential and compare against reference
         calc = EwaldCalculator(
             InversePowerLawPotential(
-                exponent=1.0,
+                exponent=1,
                 smearing=smeareff,
             ),
             lr_wavelength=smeareff / 2,