# Making an MPO from an MPS

**URL:** <https://itensor.discourse.group/t/making-an-mpo-from-an-mps/1764>\
**Category:** ITensor Julia Questions\
**Created:** [May 29, 2024, 12:15pm UTC](https://itensor.discourse.group/t/making-an-mpo-from-an-mps/1764 "2024-05-29T12:15:36Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![DavidStrachan1](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/davidstrachan1/32/165_2.png) [@DavidStrachan1](https://itensor.discourse.group/u/DavidStrachan1)\
**Post date:** [May 29, 2024, 12:15pm UTC](https://itensor.discourse.group/t/making-an-mpo-from-an-mps/1764/1 "2024-05-29T12:15:36Z")

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Hi,

I have a question regarding creating an MPO rho from an MPS psi. The way I currently do this is rho = outer(psi’,psi) but this blows up when the bond dimension of psi is above 100. Is there anyway to do this more efficiently? Or should I just increase the truncation in outer to reduce the size of rho?

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**Author:** ![miles](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/miles/32/6_2.png) [@miles](https://itensor.discourse.group/u/miles)\
**Post date:** [May 29, 2024, 5:15pm UTC](https://itensor.discourse.group/t/making-an-mpo-from-an-mps/1764/2 "2024-05-29T17:15:12Z")

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While I’m not sure that I know the perfect answer, it may depend on what you want to do after making the density matrix. What steps do you plan to take afterward? Will you be time evolving it using an open-system Lindbladian or something like that? Or are you just wanting to compute properties of the state?