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172 | #!/usr/bin/env python
#
# Simple DMRG tutorial. This code contains a basic implementation of the
# infinite system algorithm
#
# Copyright 2013 James R. Garrison and Ryan V. Mishmash.
# Open source under the MIT license. Source code at
# <https://github.com/simple-dmrg/simple-dmrg/>
# This code will run under any version of Python >= 2.6. The following line
# provides consistency between python2 and python3.
from __future__ import print_function, division # requires Python >= 2.6
# numpy and scipy imports
import numpy as np
from scipy.sparse import kron, identity
from scipy.sparse.linalg import eigsh # Lanczos routine from ARPACK
# We will use python's "namedtuple" to represent the Block and EnlargedBlock
# objects
from collections import namedtuple
Block = namedtuple("Block", ["length", "basis_size", "operator_dict"])
EnlargedBlock = namedtuple("EnlargedBlock", ["length", "basis_size", "operator_dict"])
def is_valid_block(block):
for op in block.operator_dict.values():
if op.shape[0] != block.basis_size or op.shape[1] != block.basis_size:
return False
return True
# This function should test the same exact things, so there is no need to
# repeat its definition.
is_valid_enlarged_block = is_valid_block
# Model-specific code for the Heisenberg XXZ chain
model_d = 2 # single-site basis size
Sz1 = np.array([[0.5, 0], [0, -0.5]], dtype='d') # single-site S^z
Sp1 = np.array([[0, 1], [0, 0]], dtype='d') # single-site S^+
H1 = np.array([[0, 0], [0, 0]], dtype='d') # single-site portion of H is zero
def H2(Sz1, Sp1, Sz2, Sp2): # two-site part of H
"""Given the operators S^z and S^+ on two sites in different Hilbert spaces
(e.g. two blocks), returns a Kronecker product representing the
corresponding two-site term in the Hamiltonian that joins the two sites.
"""
J = Jz = 1.
return (
(J / 2) * (kron(Sp1, Sp2.conjugate().transpose()) + kron(Sp1.conjugate().transpose(), Sp2)) +
Jz * kron(Sz1, Sz2)
)
# conn refers to the connection operator, that is, the operator on the edge of
# the block, on the interior of the chain. We need to be able to represent S^z
# and S^+ on that site in the current basis in order to grow the chain.
initial_block = Block(length=1, basis_size=model_d, operator_dict={
"H": H1,
"conn_Sz": Sz1,
"conn_Sp": Sp1,
})
def enlarge_block(block):
"""This function enlarges the provided Block by a single site, returning an
EnlargedBlock.
"""
mblock = block.basis_size
o = block.operator_dict
# Create the new operators for the enlarged block. Our basis becomes a
# Kronecker product of the Block basis and the single-site basis. NOTE:
# `kron` uses the tensor product convention making blocks of the second
# array scaled by the first. As such, we adopt this convention for
# Kronecker products throughout the code.
enlarged_operator_dict = {
"H": kron(o["H"], identity(model_d)) + kron(identity(mblock), H1) + H2(o["conn_Sz"], o["conn_Sp"], Sz1, Sp1),
"conn_Sz": kron(identity(mblock), Sz1),
"conn_Sp": kron(identity(mblock), Sp1),
}
return EnlargedBlock(length=(block.length + 1),
basis_size=(block.basis_size * model_d),
operator_dict=enlarged_operator_dict)
def rotate_and_truncate(operator, transformation_matrix):
"""Transforms the operator to the new (possibly truncated) basis given by
`transformation_matrix`.
"""
return transformation_matrix.conjugate().transpose().dot(operator.dot(transformation_matrix))
def single_dmrg_step(sys, env, m):
"""Performs a single DMRG step using `sys` as the system and `env` as the
environment, keeping a maximum of `m` states in the new basis.
"""
assert is_valid_block(sys)
assert is_valid_block(env)
# Enlarge each block by a single site.
sys_enl = enlarge_block(sys)
if sys is env: # no need to recalculate a second time
env_enl = sys_enl
else:
env_enl = enlarge_block(env)
assert is_valid_enlarged_block(sys_enl)
assert is_valid_enlarged_block(env_enl)
# Construct the full superblock Hamiltonian.
m_sys_enl = sys_enl.basis_size
m_env_enl = env_enl.basis_size
sys_enl_op = sys_enl.operator_dict
env_enl_op = env_enl.operator_dict
superblock_hamiltonian = kron(sys_enl_op["H"], identity(m_env_enl)) + kron(identity(m_sys_enl), env_enl_op["H"]) + \
H2(sys_enl_op["conn_Sz"], sys_enl_op["conn_Sp"], env_enl_op["conn_Sz"], env_enl_op["conn_Sp"])
# Call ARPACK to find the superblock ground state. ("SA" means find the
# "smallest in amplitude" eigenvalue.)
(energy,), psi0 = eigsh(superblock_hamiltonian, k=1, which="SA")
# Construct the reduced density matrix of the system by tracing out the
# environment
#
# We want to make the (sys, env) indices correspond to (row, column) of a
# matrix, respectively. Since the environment (column) index updates most
# quickly in our Kronecker product structure, psi0 is thus row-major ("C
# style").
psi0 = psi0.reshape([sys_enl.basis_size, -1], order="C")
rho = np.dot(psi0, psi0.conjugate().transpose())
# Diagonalize the reduced density matrix and sort the eigenvectors by
# eigenvalue.
evals, evecs = np.linalg.eigh(rho)
possible_eigenstates = []
for eval, evec in zip(evals, evecs.transpose()):
possible_eigenstates.append((eval, evec))
possible_eigenstates.sort(reverse=True, key=lambda x: x[0]) # largest eigenvalue first
# Build the transformation matrix from the `m` overall most significant
# eigenvectors.
my_m = min(len(possible_eigenstates), m)
transformation_matrix = np.zeros((sys_enl.basis_size, my_m), dtype='d', order='F')
for i, (eval, evec) in enumerate(possible_eigenstates[:my_m]):
transformation_matrix[:, i] = evec
truncation_error = 1 - sum([x[0] for x in possible_eigenstates[:my_m]])
print("truncation error:", truncation_error)
# Rotate and truncate each operator.
new_operator_dict = {}
for name, op in sys_enl.operator_dict.items():
new_operator_dict[name] = rotate_and_truncate(op, transformation_matrix)
newblock = Block(length=sys_enl.length,
basis_size=my_m,
operator_dict=new_operator_dict)
return newblock, energy
def infinite_system_algorithm(L, m):
block = initial_block
# Repeatedly enlarge the system by performing a single DMRG step, using a
# reflection of the current block as the environment.
while 2 * block.length < L:
print("L =", block.length * 2 + 2)
block, energy = single_dmrg_step(block, block, m=m)
print("E/L =", energy / (block.length * 2))
if __name__ == "__main__":
np.set_printoptions(precision=10, suppress=True, threshold=10000, linewidth=300)
infinite_system_algorithm(L=100, m=20)
|