Source code for netket.hilbert.spin

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# Licensed under the Apache License, Version 2.0 (the "License");
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#    http://www.apache.org/licenses/LICENSE-2.0
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from fractions import Fraction
from typing import Optional, List, Union

import numpy as np
from netket.graph import AbstractGraph
from numba import jit

from .homogeneous import HomogeneousHilbert
from ._deprecations import graph_to_N_depwarn


def _check_total_sz(total_sz, S, size):
    if total_sz is None:
        return

    local_size = 2 * S + 1

    m = round(2 * total_sz)
    if np.abs(m) > size * (2 * S):
        raise ValueError(
            "Cannot fix the total magnetization: 2|M| cannot " "exceed Nspins."
        )

    # If half-integer spins (1/2, 3/2)
    if local_size % 2 == 0:
        # Check that the total magnetization is odd if odd spins or even if even
        # number of spins
        if (size + m) % 2 != 0:
            raise ValueError(
                "Cannot fix the total magnetization: Nspins + 2*totalSz must be even."
            )
    # else if full-integer (S=1,2)
    else:
        if m % 2 != 0:
            raise ValueError(
                "Cannot fix the total magnetization to a half-integer number"
            )


@jit(nopython=True)
def _sum_constraint(x, total_sz):
    return np.sum(x, axis=1) == round(2 * total_sz)


class Spin(HomogeneousHilbert):
    r"""Hilbert space obtained as tensor product of local spin states."""

[docs] def __init__( self, s: float, N: int = 1, total_sz: Optional[float] = None, graph: Optional[AbstractGraph] = None, ): r"""Hilbert space obtained as tensor product of local spin states. Args: s: Spin at each site. Must be integer or half-integer. N: Number of sites (default=1) total_sz: If given, constrains the total spin of system to a particular value. graph: (deprecated) a graph from which to extract the number of sites. Examples: Simple spin hilbert space. >>> import netket as nk >>> hi = nk.hilbert.Spin(s=1/2, N=4) >>> print(hi.size) 4 """ N = graph_to_N_depwarn(N=N, graph=graph) local_size = round(2 * s + 1) local_states = np.empty(local_size) assert int(2 * s + 1) == local_size for i in range(local_size): local_states[i] = -round(2 * s) + 2 * i local_states = local_states.tolist() _check_total_sz(total_sz, s, N) if total_sz is not None: def constraints(x): return _sum_constraint(x, total_sz) else: constraints = None self._total_sz = total_sz if total_sz is None else total_sz self._s = s super().__init__(local_states, N, constraints)
def __pow__(self, n): if self._total_sz is None: total_sz = None else: total_sz = total_sz * n return Spin(self._s, self.size * n, total_sz=total_sz) def _mul_sametype_(self, other): assert type(self) == type(other) if self._s == other._s: if self._total_sz is None and other._total_sz is None: return Spin(s=self._s, N=self.size + other.size) return NotImplemented
[docs] def ptrace(self, sites: Union[int, List]) -> Optional["Spin"]: if isinstance(sites, int): sites = [sites] for site in sites: if site < 0 or site >= self.size: raise ValueError( f"Site {site} not in this hilbert space of site {self.size}" ) if self._total_sz is not None: raise TypeError( "Cannot take the partial trace with a total magnetization constraint." ) Nsites = len(sites) if self.size - Nsites == 0: return None else: return Spin(s=self._s, N=self.size - Nsites)
def __repr__(self): total_sz = ( ", total_sz={}".format(self._total_sz) if self._total_sz is not None else "" ) return "Spin(s={}{}, N={})".format(Fraction(self._s), total_sz, self._size) @property def _attrs(self): return (self.size, self._s, self._constraint_fn)