Line data    Source code

       1             : # SPDX-FileCopyrightText: 2025 PairInteraction Developers
       2             : # SPDX-License-Identifier: LGPL-3.0-or-later
       3           1 : from __future__ import annotations
       4             : 
       5           1 : from typing import TYPE_CHECKING, overload
       6             : 
       7           1 : from pairinteraction.perturbative.effective_system_pair import EffectiveSystemPair, EffectiveSystemPairReal
       8           1 : from pairinteraction.units import QuantityScalar
       9             : 
      10             : if TYPE_CHECKING:
      11             :     from pairinteraction.ket import KetAtom
      12             :     from pairinteraction.units import PintFloat
      13             : 
      14             : 
      15           1 : class C6(EffectiveSystemPair):
      16             :     """Class for calculating the C6 coefficient for a two-atom state ``|ket1, ket2>``.
      17             : 
      18             :     Given two `KetAtom` objects ket1 and ket2,
      19             :     this class computes the C6 coefficient for the pair state ``|ket1, ket2>``.
      20             :     This class also allows to set magnetic and electric fields similar to the `SystemAtom` class,
      21             :     as well as the angle between the two atoms like in the `SystemPair` class.
      22             : 
      23             :     Note, that ket1 and ket2 must be either the same state or states of different species.
      24             :     If you want to calculate the C6 coefficient for two different states of the same species,
      25             :     we recommend using the `EffectiveSystemPair` class with the ket_tuples subspace [(ket1, ket2), (ket2, ket1)].
      26             : 
      27             : 
      28             : 
      29             :     Examples:
      30             :         >>> import pairinteraction as pi
      31             :         >>> ket = pi.KetAtom("Rb", n=60, l=0, j=0.5, m=0.5)
      32             :         >>> c6_obj = pi.C6(ket, ket)
      33             :         >>> c6 = c6_obj.get(unit="planck_constant * GHz * micrometer^6")
      34             :         >>> print(f"{c6:.1f}")
      35             :         -138.9
      36             : 
      37             :     """
      38             : 
      39           1 :     def __init__(self, ket1: KetAtom, ket2: KetAtom) -> None:
      40           1 :         if ket1.species == ket2.species and ket1 != ket2:
      41           0 :             raise ValueError(
      42             :                 "If you want to calculate 2nd order perturbations of two different states a and b "
      43             :                 "(of the same species), "
      44             :                 "please use the EffectiveSystemPair([(a,b), (b,a)]) class."
      45             :             )
      46             : 
      47           1 :         super().__init__([(ket1, ket2)])
      48           1 :         self.set_perturbation_order(2)
      49             : 
      50             :         # Set some default distance. The exact value does not matter for the C6 value,
      51             :         # but too small distances result in warnings about the basisvec admixtures
      52           1 :         self.set_distance(100, unit="micrometer")
      53             : 
      54             :     @overload
      55             :     def get(self, unit: None = None) -> PintFloat: ...
      56             : 
      57             :     @overload
      58             :     def get(self, unit: str) -> float: ...
      59             : 
      60           1 :     def get(self, unit: str | None = None) -> float | PintFloat:
      61             :         r"""Get the C6 coefficient of the interaction between the specified ket1 and ket2.
      62             : 
      63             :         The C6 coefficient is defined such that the van der Waals interaction potential is given by
      64             :         :math:`V(r) = -C_6/r^6`.
      65             :         Note, the entry in the effective Hamiltonian matrix (see :meth:`get_effective_hamiltonian`)
      66             :         has the opposite sign as the C6 coefficient.
      67             : 
      68             :         Args:
      69             :             unit: The unit in which to return the C6 coefficient.
      70             :                 If None, returns a pint object.
      71             : 
      72             :         """
      73           1 :         h_eff_pint = self.get_effective_hamiltonian(return_order=2)
      74           1 :         distance = self.system_pair.get_distance()
      75           1 :         c6_pint = -h_eff_pint[0, 0] * distance**6  # type: ignore [index]  # pint does not know it can be indexed
      76           1 :         return QuantityScalar.convert_pint_to_user(c6_pint, "c6", unit)
      77             : 
      78             : 
      79           1 : class C6Real(C6, EffectiveSystemPairReal):
      80           1 :     pass