# Using LorentzVector/PtEtaPhiM

Hello ROOT friends
I am trying to work with LorentzVector and am struggling to set it up properly.
From the documentation, I declare the vector as:

``````LorentzVector<PtEtaPhiM4D<double>> vectorName;
``````

With

``````include <Math/Vector4D.h>
``````

At the top.
This says the Lorentz vector is undefined “error: ‘LorentzVector’ was not declared in this scope”.

``````include <LorentzVector.h>
``````

Which it then says does not exist “LorentzVector.h: No such file or directory”.

So what is the trick for this?
Thanks!

EDIT: I’ll also add I would like to get the delta phi between this and another vector. I know for TLorentz vector there was a function for this

``````TLorentzVector.DeltaPhi(TLorentzVector2);
``````

but I don’t see such a function for Lorentz Vector, so what should I do?

Try with

``````#include <Math/LorentzVector.h>
#include <Math/PtEtaPhiM4D.h>

ROOT::Math::LorentzVector<ROOT::Math::PtEtaPhiM4D<double>> vectorName;
``````

Thanks, this worked for initiating the vector.
Any idea for the deltaPhi? or should I just do that manually?

Maybe using ROOT::Math::VectorUtil::DeltaPhi