## Learn to Use ITensor

# Version 2 to 3 Upgrade Guide

Here we list some of the largest and most important changes to know about when switching from ITensor version 2 to version 3. We also discuss some upgrades for specific tasks you may have, such as a DMRG calculation.

For much more detailed info about the changes made in version 3, see the changelog.

To move to version 3 if you have already cloned ITensor,
you have to switch to the `v3`

branch. To do so, use the commands

`git pull`

`git checkout v3`

## Major or Required Changes

**C++17 is required to compile the ITensor Library.**Switching to C++17 allows us to make significant interface improvements, such as using multiple return values, and also make internal, developer-level code easier to read and maintain. To upgrade, change your compiler flags

from`-std=c++11`

to`-std=c++17`

. (For C++ aficionados, here is a website with the new C++17 features.)**Changes to Index objects:**- only the dimension is required to construct an Index (no name string)
- you can optionally provide a string which is a comma-separated list of "tags" Index objects must have the same set of tags to compare equal. Tags can be added, removed, and replaced.
- the dimension of an Index i is now accessed as
`dim(i)`

, versus`i.m()`

in version 2.

Click to Show Exampleauto i = Index(3); println("i is an Index of dimension ",dim(i)); //prints: i is an Index of dimension 3 auto j = Index(5,"j,Site,Top"); println("j has the tags ",tags(j)); //prints: j has the tags j,Top,Site

**The**An IQIndex is now just an Index which carries extra quantum number (QN) information. An IQTensor is now just an ITensor with block-sparse storage internally and whose indices carry quantum numbers. You can use the`IQTensor`

,`IQIndex`

,`IQMPS`

, and`IQMPO`

types have been removed.`hasQNs`

function to inspect whether an Index or ITensor has QN block structure.Click to Show Exampleauto i = Index(QN({"Sz",-1}),4, QN({"Sz", 0}),8, QN({"Sz",+1}),4); Print(hasQNs(i)); //prints: hasQNs(i) = true println("i is an Index of dimension ",dim(i)); //prints: i in an Index of dimension 16 auto T = ITensor(i,prime(i)); T.set(i=1,prime(i)=1,-0.234); PrintData(T); //view the storage of T to see it's block-sparse Print(hasQNs(T)); //print: hasQNs(T) = true

**Quantum number QN objects use strings to label each of their values.**Each sector of a QN object is specified by a string and an integer value. You can optionally specify a "mod factor" N if the sector follows a @@\mathbb{Z}_N@@ addition rule. For efficiency, the name string of the sector must be seven characters or less. Sectors are sorted by their name and you must use the name to access the value. Having strings in QN objects allows sensible addition between QNs which do not all carry the same sectors; a missing sector is treated as having the value zero. For more information see the QN docs.Click to Show Exampleauto q1 = QN({"Sz",+1}); Print(q1.val("Sz")); //prints: q1.val("Sz") = 1 auto q2 = QN({"N",3},{"T",-2}); Print(q2.val("T")); //prints: q2.val("T") = -2 Print(q2.val("N")); //prints: q2.val("N") = 3 //Make a QN with a Z2 addition rule: auto q3 = QN({"P",1,2}); Print(q3.val("P")); //prints: q3.val("P") = 1 Print(q3.mod("P")); //prints: q3.mod("P") = 2 Print(q3+q3); //prints: q3+q3 = QN({"P",0});

## Recommended, Optional Changes

These are changes we recommend to follow the standards of version 3, or to avoid using now-deprecated features, but which are not required to make your code compile:

**To retrieve elements of tensors**use the free function`elt`

if the ITensor is real, or`eltC`

if the ITensor could be complex.Click to Show Exampleauto T = randomITensor(i,j,k); auto x = elt(T,i=1,j=1,k=1); auto V = randomITensorC(m,n); auto z = eltC(V,n=2,m=3);

**Physics-specific Site Sets Carry QNs by Default**. If you use a site set such as`SpinHalf`

or`Electron`

(formerly called "Hubbard"), the indices and operators produced from these will be QN-block-sparse. If you wish to omit or not have the QN sparsity, pass a named argument`{"ConserveQNs=",false}`

to the site set constructor, for example

`auto sites = SpinHalf(N,{"ConserveQNs=",false});`

.**Tensor decompositions provide multiple return values.**The new, preferred interfaces for tensor decompositions such as`svd`

and`diagHermitian`

no longer takes the factored results by reference, but returns them using the new "structured binding" or multiple-return-value feature of C++17. For more details, see the tensor decomposition docs.Click to Show Exampleauto l1 = Index(8,"l1"); auto l2 = Index(4,"l2"); auto s = Index(2,"s1"); auto T = randomITensor(l1,s,l2); auto [U,S,V] = svd(T,{l1,s});

**To access individual MPS tensors**, say of an MPS object`psi`

, just call`psi(j)`

. To replace the tensor at site j with a tensor`T`

, call`psi.set(j,T)`

. Or to modify the tensor in-place, do`psi.ref(j) = T`

.

## Task-Specific Upgrades

**Upgrading a DMRG Calculation**The following steps should be sufficient for upgrading an existing DMRG code from version 2. Also we suggest you look at the sample DMRG codes in the sample/ folder distributed with the ITensor source.- if using AutoMPO to construct your Hamiltonian MPO, replace the line
`auto H = MPO(ampo);`

with`auto H = toMPO(ampo);`

- if using IQMPO and IQMPS, just replace these with
`MPO`

and`MPS`

instead and make sure the indices or site set you use to construct these carry QN block structure (you can print out these objects to see the QNs and ITensor storage type). - make sure to initialize the MPS you pass as an initial state to the
`dmrg`

function. In version 2, a non-QN MPS would be randomly initialized, but now you must initialize all MPS. See the sample/dmrg.cc code for an example of initializing an MPS to a particular product state. - when constructing a Sweeps object, replace the line
`sweeps.maxm() = 10,20,40;`

with`sweeps.maxdim() = 10,20,40;`

- prefer to call dmrg as
`auto [energy,psi] = dmrg(H,psi0,sweeps,"Quiet");`

- if using AutoMPO to construct your Hamiltonian MPO, replace the line
**Changes to priming functions**To accommodate the new tags interface, some priming functions have been superceded by tag functions (since the prime level can be accessed through the new tag interface). Please see the**Tag and Prime Methods**section of the IndexSet docs for more details on the new interface. This interface works for ITensor, MPS and MPO objects. For example:- instead of
`mapprime(T,0,1)`

, use`replaceTags(T,"0","1")`

(note that tags that are just integer numbers are interpreted as prime levels). - instead of
`swapPrime(T,0,1)`

, use`swapTags(T,"0","1")`

. - for all tagging and priming methods, optional matching tags and indices are the last
input of the function (for example, use
`prime(T,2,i)`

to increase the prime level of Index`i`

of ITensor`T`

by two). - indices used for matching are now always matched exactly, without ignoring the prime level.
For example, if ITensor
`auto T = ITensor(i,prime(i),j)`

where Index`i`

is`auto i = Index(2,"i")`

and Index`j`

is`Index(3,"j")`

, to prime indices`i`

and`prime(i)`

use either`prime(T,{i,prime(i)})`

or`prime(T,"i")`

since`prime(T,i)`

will only prime Index`i`

).

- instead of
**Changes to MPS and MPO Functions**Some conventions and names have changed for common MPS and MPO functions, such as`applyMPO`

and`nmultMPO`

. For more details, please see the MPS and MPO docs.- Use
`removeQNs`

to remove the QNs of an MPS or MPO, instead of converting from IQMPS to MPS or IQMPO to MPO. - use
`inner`

and`innerC`

instead of`overlap`

and`overlapC`

to get inner products of MPSs (and inner products of MPSs contracted with MPOs). - use
`trace`

and`traceC`

to get the trace of an MPO or the trace of the product of two MPOs (superceding some use cases of`overlap`

). - the interfaces
`exactApplyMPO`

and`fitApplyMPO`

have been removed in favor of the single`applyMPO`

function. - when calling
`auto y = applyMPO(A,x)`

, for MPS`x`

with unprimed site indices and MPO`A`

with pairs of prime and unprimed site indices, the resulting MPS`y`

will have primed indices (or in general, the site indices that are not shared by MPO`A`

and`x`

). Use`y.replaceTags("1","0")`

or`y.noPrime()`

to get back an MPS with unprimed site indices. - for MPOs
`A`

and`B`

with pairs of primed and unprimed site indices, contract them together with`auto C = nmultMPO(prime(A),B)`

. The inputs must share one site index per tensor, and the output MPO`C`

will have the remaining unshared site indices (so one unprimed site index and one site index of prime level 2). One can use`C.setPrime(1,"2")`

or`C.replaceTags("2","1")`

to get an MPO`C`

with pairs of unprimed and single-primed indices.

- Use

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