# AutoMPO

## Description

ITensors.AutoMPOType

An AutoMPO stores a collection of operator terms, to be later summed together into an MPO by calling the function MPO on the AutoMPO object. Each term is a product of local operators specified by names such as "Sz" or "N", times an optional coefficient which can be real or complex.

Which local operator names are available is determined by the function op associated with the TagType defined by special Index tags, such as "S=1/2","S=1", "Fermion", and "Electron".

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## Methods

ITensors.add!Function
add!(ampo::AutoMPO,
op1::String, i1::Int)

coef::Number,
op1::String, i1::Int)

op1::String, i1::Int,
op2::String, i2::Int,
ops...)

coef::Number,
op1::String, i1::Int,
op2::String, i2::Int,
ops...)

+(ampo:AutoMPO, term::Tuple)

Add a single- or multi-site operator term to the AutoMPO ampo. Each operator is specified by a name (String) and a site number (Int). The second version accepts a real or complex coefficient.

The + operator version of this function accepts a tuple with entries either (String,Int,String,Int,...) or (Number,String,Int,String,Int,...) where these tuple values are the same as valid inputs to the add! function. For inputting a very large number of terms (tuples) to an AutoMPO, consider using the broadcasted operator .+= which avoids reallocating the AutoMPO after each addition.

Examples

ampo = AutoMPO()

ampo += ("Sz",3,"Sz",4)

ampo += (0.5,"S+",4,"S-",5)

ampo .+= (0.5,"S+",5,"S-",6)
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ITensors.MPOMethod
MPO(ampo::AutoMPO,sites::Vector{<:Index};kwargs...)

Convert an AutoMPO object ampo to an MPO, with indices given by sites. The resulting MPO will have the indices sites[1], sites[1]', sites[2], sites[2]' etc. The conversion is done by an algorithm that compresses the MPO resulting from adding the AutoMPO terms together, often achieving the minimum possible bond dimension.

Examples

ampo = AutoMPO()
ampo += ("Sz",1,"Sz",2)
ampo += ("Sz",2,"Sz",3)
ampo += ("Sz",3,"Sz",4)

sites = siteinds("S=1/2",4)
H = MPO(ampo,sites)
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