## Learn to Use ITensor

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# Creating a set of Site indices to use as a lattice

To begin most calculations you need a set of lattice sites to define your Hilbert space. Although the ITensor library defines a set of SiteSet classes which automate this for you, there could be a lot of reasons why you would want to do it yourself. Below is a simple pattern you can use to create a set of site Indices and save them in an IndexSet, a container for a set of indices used throughout ITensor.

auto N = 100; //number of sites
auto d = 2; //dimension of local Hilbert space of each site
auto sites = IndexSet(N);
for( auto n : range1(N) )
sites(n) = Index(d,"Site,n="+str(n));

//Now we can use these sites, for example, to make tensors
//such as operators

//For convenience, save the third index and its primed
//version
auto s3 = sites(3);
auto s3P = prime(s3);

auto sz3 = ITensor(s3,s3P);
sz3.set(s3=1,s3P=1, 0.5);
sz3.set(s3=2,s3P=2,-0.5);


Complete sample code:

#include "itensor/all.h"

using namespace itensor;

int main()
{
auto N = 100; //number of sites
auto d = 2; //dimension of local Hilbert space of each site
auto sites = IndexSet(N);
for( auto n : range1(N) )
sites(n) = Index(d,"Site,n="+str(n));

auto s3 = sites(3);
auto s3P = prime(s3);
auto sz3 = ITensor(s3,s3P);
sz3.set(s3=1,s3P=1, 0.5);
sz3.set(s3=2,s3P=2,-0.5);

return 0;
}


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