## Introduction

#### Clone via github.

(Preferred method—see our git quickstart guide.)

Or download latest

version v1.0.5
(changelog)

ITensor—Intelligent Tensor—is a C++ library for implementing tensor product wavefunction calculations.
It is efficient and flexible enough to be used for research-grade simulations.

Features include:

- A complete DMRG code
- Quantum number conserving tensors
- Efficient matrix product state types
- Complex numbers (handled lazily: no efficiency loss if real)

## Recent News

- ITensor Version 1.0
- Article: Should you use Periodic Boundary Conditions in DMRG?
- ITensor at 2014 Sherbrooke Summer School

ITensors are as easy to multiply as scalars: matching indices automatically contract, making it simple to transcribe tensor network diagrams into correct, efficient code.

For example, the diagram below can be converted to code as

**Getting started with ITensor:**

- Clone the latest version:

Or download if you do not have git.`git clone https://github.com/ITensor/ITensor itensor`

(Cloning with the "git" program allows tracking changes to ITensor; for more see our git quickstart guide.) - Read the installation instructions.
- Learn more from the documentation.

## Code Samples

## Perform a DMRG Calculation

SpinOne sites(100); //define Hilbert space of 100 spin-lattice sites MPO H = Heisenberg(sites); //pre-defined Hamiltonian; easy to write your own MPS psi(sites); Sweeps sweeps(5); //perform 5 sweeps of DMRG sweeps.maxm() = 50, 50, 100, 100, 200; //max number of states kept each sweep dmrg(psi,H,sweeps); //run the DMRG algorithm Real energy = psiHphi(psi,H,psi); //can analyze wavefunction afterwards

## Multiply Two ITensors

Index a("a",2), b("b",2), c("c",2); ITensor Z(a,b), X(c,b); commaInit(Z,a,b) = 1, 0, 0,-1; commaInit(X,b,c) = 0, 1, 1, 0; //the * operator finds and //contracts common index 'b' //regardless of index order: ITensor R = Z * X; Print( R(a(1),c(2)) ); //Prints: R(a(1),c(2)) = 1 Print( R(a(2),c(1)) ); //Prints: R(a(1),c(2)) = -1