# DMRG and Numerical Methods **URL:** https://itensor.discourse.group/c/dmrg-and-numerical-methods/8.md?page=1 [Latest](https://itensor.discourse.group/latest.md) · [Categories](https://itensor.discourse.group/categories.md) · [Tags](https://itensor.discourse.group/tags.md) **Page:** 2 --- ## [Calculate dynamic correlation function for fermonic operators in ITensor](https://itensor.discourse.group/t/calculate-dynamic-correlation-function-for-fermonic-operators-in-itensor/2476)
**Author:** [@ZhouGD2000](https://itensor.discourse.group/u/ZhouGD2000)\ **Replies:** 1\ **Last updated:** [August 24, 2025, 8:48am UTC](https://itensor.discourse.group/t/calculate-dynamic-correlation-function-for-fermonic-operators-in-itensor/2476 "2025-08-24T08:48:43Z")
Hi, I would like to calculate the dynamic correlation function, e.g., \\langle c(t)c^{\\dagger}(0)\\rangle for fermionic operators by ITensor. Standard way to this using time evolution is calculate the groundstate |\\psi… --- ## [Entanglement entropy from DMRG for XXZ model](https://itensor.discourse.group/t/entanglement-entropy-from-dmrg-for-xxz-model/2464)
**Author:** [@sou27](https://itensor.discourse.group/u/sou27)\ **Replies:** 2\ **Last updated:** [August 7, 2025, 11:18pm UTC](https://itensor.discourse.group/t/entanglement-entropy-from-dmrg-for-xxz-model/2464 "2025-08-07T23:18:24Z")
As the title indicates, I was playing around with the DMRG module for the (OBC) XXZ model, shown below H = \\sum \\sigma^x\_i\\sigma^x\_{i+1} + \\sigma^y\_i\\sigma^y\_{i+1} - \\Delta\\sigma^z\_i\\sigma^z\_{i+1} In particular, when I… --- ## [Exited state with fixed QN](https://itensor.discourse.group/t/exited-state-with-fixed-qn/2449)
**Author:** [@jackaraz](https://itensor.discourse.group/u/jackaraz)\ **Replies:** 2\ **Last updated:** [July 22, 2025, 7:59pm UTC](https://itensor.discourse.group/t/exited-state-with-fixed-qn/2449 "2025-07-22T19:59:16Z")
Hi, I’m trying to prepare an excited state that satisfies a certain quantum number at a flavour sector. Normally, I use conserve\_qns=True with the hamiltonian ampo = AutoMPO() sites = siteinds("Fermion", nsites\*nflavour… --- ## [How to build an MPO using 2-sites local operators?](https://itensor.discourse.group/t/how-to-build-an-mpo-using-2-sites-local-operators/2445)
**Author:** [@Joacop16](https://itensor.discourse.group/u/Joacop16)\ **Replies:** 1\ **Last updated:** [July 17, 2025, 10:39pm UTC](https://itensor.discourse.group/t/how-to-build-an-mpo-using-2-sites-local-operators/2445 "2025-07-17T22:39:56Z")
Hi, ITensor Teams！ For a work on Lindblad Equations I need to build an MPO adding non-local terms. Without entering in so much details please consider the following example: H = \\sum\_{i = 1}^{N-1}\\sigma\_{x}\\sigma\_{x+1} … --- ## [Dynamical Structure Factor of 1D Spin-1 chain](https://itensor.discourse.group/t/dynamical-structure-factor-of-1d-spin-1-chain/2440)
**Author:** [@SS\_Maitra](https://itensor.discourse.group/u/SS_Maitra)\ **Replies:** 1\ **Last updated:** [July 12, 2025, 11:07am UTC](https://itensor.discourse.group/t/dynamical-structure-factor-of-1d-spin-1-chain/2440 "2025-07-12T11:07:12Z")
Hi! I am trying to calculate the dynamical structure factor of 1D spin-1 chain using the following relation: S^{zz}(k,\\omega) = \\int dt e^{-i\\omega t} \\sum\_{j} e^{-ikr\_j} \\langle\\psi\_0| S^z\_j(t)S^z\_0|\\psi\_0\\rangle H… --- ## [Using ITensorMPS to efficiently calculate all principal minors of a matrix](https://itensor.discourse.group/t/using-itensormps-to-efficiently-calculate-all-principal-minors-of-a-matrix/2435)
**Author:** [@Fred](https://itensor.discourse.group/u/Fred)\ **Replies:** 0\ **Last updated:** [July 8, 2025, 1:07pm UTC](https://itensor.discourse.group/t/using-itensormps-to-efficiently-calculate-all-principal-minors-of-a-matrix/2435 "2025-07-08T13:07:10Z")
Hi! I come from statistical and quantum physics and try to calculate all 2^M principle minors (PMs) of a real M\\times M matrix G using the method described by Griffin & Tsatsomeros, see also this mathoverflow posting. B… --- ## [Is it possible to limit disk usage when using write\_when\_maxdim\_exceeded=true in DMRG?](https://itensor.discourse.group/t/is-it-possible-to-limit-disk-usage-when-using-write-when-maxdim-exceeded-true-in-dmrg/2427)
**Author:** [@gjduan](https://itensor.discourse.group/u/gjduan)\ **Replies:** 0\ **Last updated:** [June 25, 2025, 2:29am UTC](https://itensor.discourse.group/t/is-it-possible-to-limit-disk-usage-when-using-write-when-maxdim-exceeded-true-in-dmrg/2427 "2025-06-25T02:29:53Z")
Hi all, I’m running DMRG simulations using ITensors.jl, and some of my systems require a very large bond dimension — so large that the memory usage exceeds the capacity of a single node (about 1 TB RAM). I’m using the w… --- ## [DMRG not converging for FHM in SC phase](https://itensor.discourse.group/t/dmrg-not-converging-for-fhm-in-sc-phase/2417)
**Author:** [@hmargasak](https://itensor.discourse.group/u/hmargasak)\ **Replies:** 2\ **Last updated:** [June 10, 2025, 11:30pm UTC](https://itensor.discourse.group/t/dmrg-not-converging-for-fhm-in-sc-phase/2417 "2025-06-10T23:30:51Z")
ITensor team, I am trying to measure various observables of the extended fermi hubbard model’s ground state in the singlet superconducting phase. I’m having a simple but puzzling problem: DMRG is not converging, and the… --- ## [Exact Diagonalization code's link not working](https://itensor.discourse.group/t/exact-diagonalization-codes-link-not-working/2416)
**Author:** [@Jyoti\_Bisht](https://itensor.discourse.group/u/Jyoti_Bisht)\ **Replies:** 11\ **Last updated:** [June 9, 2025, 5:51pm UTC](https://itensor.discourse.group/t/exact-diagonalization-codes-link-not-working/2416 "2025-06-09T17:51:20Z")
https://github.com/ITensor/ITensors.jl/blob/main/examples/exact\_diagonalization/exact\_diagonalization.jl This is the link given on this site for exact diagonalization through ITensor, but now this link is not working. C… --- ## [First DMRG sweep takes significantly Longer](https://itensor.discourse.group/t/first-dmrg-sweep-takes-significantly-longer/2410)
**Author:** [@BenRClark](https://itensor.discourse.group/u/BenRClark)\ **Replies:** 3\ **Last updated:** [June 3, 2025, 11:25pm UTC](https://itensor.discourse.group/t/first-dmrg-sweep-takes-significantly-longer/2410 "2025-06-03T23:25:57Z")
Hello, I have been experimenting with ITensor’s DMRG so I can have easy quantum number conservation. Previously, I was using QUIMB to do DMRG, and the DMRG algorithm used there could find the ground state significantly … --- ## [Understanding the METTS implementation in iTensor (example in repository is dead)](https://itensor.discourse.group/t/understanding-the-metts-implementation-in-itensor-example-in-repository-is-dead/2412)
**Author:** [@Ritvik1sharma](https://itensor.discourse.group/u/Ritvik1sharma)\ **Replies:** 1\ **Last updated:** [June 3, 2025, 9:54pm UTC](https://itensor.discourse.group/t/understanding-the-metts-implementation-in-itensor-example-in-repository-is-dead/2412 "2025-06-03T21:54:21Z")
Hi i am trying to switch my code base to switch from direct sampling method (where for every element in the system, I take a trace over all other sites and find the most likely assignement for my current index and procee… --- ## [To understand if an MPS is VBS?](https://itensor.discourse.group/t/to-understand-if-an-mps-is-vbs/2395)
**Author:** [@shuklavaibhav](https://itensor.discourse.group/u/shuklavaibhav)\ **Replies:** 5\ **Last updated:** [May 27, 2025, 2:57pm UTC](https://itensor.discourse.group/t/to-understand-if-an-mps-is-vbs/2395 "2025-05-27T14:57:13Z")
I have solved the 2D Sandvik model (J-Q model) using DMRG and found the ground state which is an MPS. But I need to verify if this MPS is a VBS(valence bond solid) how do I do that? --- ## [negative energy for a semi positive definite (non-local) hamiltonian](https://itensor.discourse.group/t/negative-energy-for-a-semi-positive-definite-non-local-hamiltonian/2392)
**Author:** [@prange](https://itensor.discourse.group/u/prange)\ **Replies:** 2\ **Last updated:** [May 22, 2025, 12:50am UTC](https://itensor.discourse.group/t/negative-energy-for-a-semi-positive-definite-non-local-hamiltonian/2392 "2025-05-22T00:50:54Z")
I am trying to evaluate the ground state wave function of a semi positive hamiltonian, i.e., H= L^\\dagger L, with L non-hermitian and sum of local terms. However, I find that the energy in the dmrg can be negative. Do y… --- ## [Understanding conserved QNs](https://itensor.discourse.group/t/understanding-conserved-qns/2384)
**Author:** [@ElevenBadger](https://itensor.discourse.group/u/ElevenBadger)\ **Replies:** 2\ **Last updated:** [May 9, 2025, 8:51pm UTC](https://itensor.discourse.group/t/understanding-conserved-qns/2384 "2025-05-09T20:51:57Z")
Hi all, I’m trying to study a multi-component Hubbard model, similar to what was asked before here. However, instead of splitting up the sites into a mixed “Electron” space, I’m trying to define a single custom SiteType… --- ## [DMRG on multiple GPUs](https://itensor.discourse.group/t/dmrg-on-multiple-gpus/2373)
**Author:** [@Liftuck](https://itensor.discourse.group/u/Liftuck)\ **Replies:** 0\ **Last updated:** [May 5, 2025, 11:49am UTC](https://itensor.discourse.group/t/dmrg-on-multiple-gpus/2373 "2025-05-05T11:49:19Z")
Hi, I have 2 questions I hope some of you might be able to help me with. I am currently simulating QLMs using DMRG. I have recently made the switch to GPU (this was very easy, thanks to the devs for that) but due to an… --- ## [A problem of MPS self - contraction.](https://itensor.discourse.group/t/a-problem-of-mps-self-contraction/2366)
**Author:** [@UynajGI](https://itensor.discourse.group/u/UynajGI)\ **Replies:** 4\ **Last updated:** [May 3, 2025, 9:21am UTC](https://itensor.discourse.group/t/a-problem-of-mps-self-contraction/2366 "2025-05-03T09:21:06Z")
Hello everyone. I have a simple question, but I haven’t found a suitable implementation method for the time being (using the Julia version): I expect to perform a self - contraction of the j - th site and the j + N/2 -… --- ## [Numerical evidence for even-odd width effects in 2D repulsive Hubbard model (cylinder) at half-filling: Spin correlations and Haldane conjecture connection?](https://itensor.discourse.group/t/numerical-evidence-for-even-odd-width-effects-in-2d-repulsive-hubbard-model-cylinder-at-half-filling-spin-correlations-and-haldane-conjecture-connection/2335)
**Author:** [@swanchristmas](https://itensor.discourse.group/u/swanchristmas)\ **Replies:** 1\ **Last updated:** [May 2, 2025, 11:03am UTC](https://itensor.discourse.group/t/numerical-evidence-for-even-odd-width-effects-in-2d-repulsive-hubbard-model-cylinder-at-half-filling-spin-correlations-and-haldane-conjecture-connection/2335 "2025-05-02T11:03:06Z")
I’m investigating the spin-spin correlation behavior of the repulsive 2D Hubbard model at half-filling under a narrow cylinder geometry (periodic boundary in circumferential direction, open in axial direction). Theoretic… --- ## [Total spin operator for Electron sites system](https://itensor.discourse.group/t/total-spin-operator-for-electron-sites-system/2358)
**Author:** [@steffen](https://itensor.discourse.group/u/steffen)\ **Replies:** 5\ **Last updated:** [April 28, 2025, 9:51am UTC](https://itensor.discourse.group/t/total-spin-operator-for-electron-sites-system/2358 "2025-04-28T09:51:42Z")
Hi there, first of all thanks a lot for developing this great package for Julia, it’s really powerful and simple to use (from my limited experience)! I’m considering a Hubbard model for a 1D ladder system (i.e. 2xN geo… --- ## [Example codes for using QTT to solve a PDE](https://itensor.discourse.group/t/example-codes-for-using-qtt-to-solve-a-pde/2344)
**Author:** [@sungjuGit](https://itensor.discourse.group/u/sungjuGit)\ **Replies:** 2\ **Last updated:** [April 15, 2025, 4:19pm UTC](https://itensor.discourse.group/t/example-codes-for-using-qtt-to-solve-a-pde/2344 "2025-04-15T16:19:58Z")
Hello, I am a complete novice to ITensors and I am trying to learn to use MPS/MPO to solve PDEs (not necessarily of quantum mechanics origin). I was wondering if there are simple but hopefully complete example codes on… --- ## [Unexpected scaling of OpSum](https://itensor.discourse.group/t/unexpected-scaling-of-opsum/2333)
**Author:** [@Bernardo](https://itensor.discourse.group/u/Bernardo)\ **Replies:** 7\ **Last updated:** [April 15, 2025, 9:59am UTC](https://itensor.discourse.group/t/unexpected-scaling-of-opsum/2333 "2025-04-15T09:59:31Z")
Hello! I am trying to build a MPO which includes 4-sites non-local interaction. The first thing I do is define a OpSum and add all the terms I want. Given that it’s a 4 site interaction, I expect the time to construct t… --- ## [On computing Neutral Gap using OBC](https://itensor.discourse.group/t/on-computing-neutral-gap-using-obc/2330)
**Author:** [@lmartinsverissimo](https://itensor.discourse.group/u/lmartinsverissimo)\ **Replies:** 1\ **Last updated:** [April 6, 2025, 1:42pm UTC](https://itensor.discourse.group/t/on-computing-neutral-gap-using-obc/2330 "2025-04-06T13:42:09Z")
Hello ITensor Team, First, thanks for the effort and development of such a fantastic package. I’m working in a 4-spins cell ferrimagnetic model that presents two Ising universality class transitions. I’m having some is… --- ## [Difference in observable value for spins with fully symmetric Hamiltonian](https://itensor.discourse.group/t/difference-in-observable-value-for-spins-with-fully-symmetric-hamiltonian/2332)
**Author:** [@ray-l](https://itensor.discourse.group/u/ray-l)\ **Replies:** 1\ **Last updated:** [April 4, 2025, 2:38pm UTC](https://itensor.discourse.group/t/difference-in-observable-value-for-spins-with-fully-symmetric-hamiltonian/2332 "2025-04-04T14:38:14Z")
Hello, This is more of a conceptual question, and I think I might know the answer, but I’m still interested in knowing what you guys think. I have discovered that very often, even with fermionic Hamiltonian that’s symm… --- ## [Multi-point correlation function using ITensor DMRG](https://itensor.discourse.group/t/multi-point-correlation-function-using-itensor-dmrg/2238)
**Author:** [@ZAHID-8017](https://itensor.discourse.group/u/ZAHID-8017)\ **Replies:** 5\ **Last updated:** [March 24, 2025, 7:29am UTC](https://itensor.discourse.group/t/multi-point-correlation-function-using-itensor-dmrg/2238 "2025-03-24T07:29:09Z")
Dear All, ITensor provides a function that computes correlation function matrix for two operators A\_{i}, B\_{i}. That is C\[i, j\] = correlation\_matrix(psi, A\_{i}, B\_{i}). How can I compute a multi-point correlation quan… --- ## [TDVP with Penatly Terms](https://itensor.discourse.group/t/tdvp-with-penatly-terms/2222)
**Author:** [@Liftuck](https://itensor.discourse.group/u/Liftuck)\ **Replies:** 3\ **Last updated:** [March 14, 2025, 10:24pm UTC](https://itensor.discourse.group/t/tdvp-with-penatly-terms/2222 "2025-03-14T22:24:41Z")
I am currently working on Quantum Link Models on 2D lattices using DMRG. This works pretty well so far. I am using a penalty term to make sure that gauß-law is fullfilled. I would like to now do time evolution. For this… --- ## [Expectation value of an operator after dmrg using ground state.](https://itensor.discourse.group/t/expectation-value-of-an-operator-after-dmrg-using-ground-state/2284)
**Author:** [@shuklavaibhav](https://itensor.discourse.group/u/shuklavaibhav)\ **Replies:** 15\ **Last updated:** [March 11, 2025, 6:00pm UTC](https://itensor.discourse.group/t/expectation-value-of-an-operator-after-dmrg-using-ground-state/2284 "2025-03-11T18:00:08Z")
Hello there to the community hope all are doing good, I have done a DMRG calculation of 1d spin 1/2 Heisenberg model and found its ground state as an mps, now I wish to calculate some expectations like Sx, Sy, Sz, S.S .… --- ## [Non-hermitian DMRG algorithm for steady state](https://itensor.discourse.group/t/non-hermitian-dmrg-algorithm-for-steady-state/2288)
**Author:** [@qkrcksqls135](https://itensor.discourse.group/u/qkrcksqls135)\ **Replies:** 8\ **Last updated:** [March 9, 2025, 8:10am UTC](https://itensor.discourse.group/t/non-hermitian-dmrg-algorithm-for-steady-state/2288 "2025-03-09T08:10:18Z")
Hi. I’m trying to get a most steady state (update MPS in DMRG sweep with eigenvector corresponding the lowest “imaginary part” eigenvalue) in Non-hermitian Hamiltonian with ITensor DMRG. I already know about ishermitian… --- ## [Absence of a feedback in case of non hermitian hamiltonian](https://itensor.discourse.group/t/absence-of-a-feedback-in-case-of-non-hermitian-hamiltonian/2279)
**Author:** [@VinceNeede](https://itensor.discourse.group/u/VinceNeede)\ **Replies:** 21\ **Last updated:** [March 7, 2025, 1:35pm UTC](https://itensor.discourse.group/t/absence-of-a-feedback-in-case-of-non-hermitian-hamiltonian/2279 "2025-03-07T13:35:24Z")
This Issue has arise from this previous question MPO for Anyon Hubbard model, the user in question wrote a non hermitian hamiltonian by mistake, but he was able to notice that thanks to a warning from KrylovKit. When I t… --- ## [MPS purification and MPO purification](https://itensor.discourse.group/t/mps-purification-and-mpo-purification/2283)
**Author:** [@Q-SF](https://itensor.discourse.group/u/Q-SF)\ **Replies:** 0\ **Last updated:** [March 5, 2025, 8:34am UTC](https://itensor.discourse.group/t/mps-purification-and-mpo-purification/2283 "2025-03-05T08:34:51Z")
Hi! There are two types of finite-temperature purification codes: MPO purification and MPS purification. I have four questions regarding these two codes. What I want to calculate is the dynamical structure factor of the… --- ## [MPO for Anyon Hubbard model](https://itensor.discourse.group/t/mpo-for-anyon-hubbard-model/2275)
**Author:** [@Ashinxinl](https://itensor.discourse.group/u/Ashinxinl)\ **Replies:** 11\ **Last updated:** [March 6, 2025, 6:12am UTC](https://itensor.discourse.group/t/mpo-for-anyon-hubbard-model/2275 "2025-03-06T06:12:06Z")
Hello everyone, I am trying to reproduce the results from a paper on the Anyon Hubbard model. (ImageSource:DOI: 10.1103/PhysRevB.97.115126) After defining the operators, I encountered an issue: when summing terms … --- ## [On the naive contraction implementation](https://itensor.discourse.group/t/on-the-naive-contraction-implementation/2271)
**Author:** [@VinceNeede](https://itensor.discourse.group/u/VinceNeede)\ **Replies:** 0\ **Last updated:** [February 28, 2025, 9:16am UTC](https://itensor.discourse.group/t/on-the-naive-contraction-implementation/2271 "2025-02-28T09:16:45Z")
Hello everyone, In the current implementation of the naive contraction algorithm of MPS with MPO, a new MPS is computed from contracting each tensor of the MPS with the corresponding tensor of the MPO, then this new sta… [Previous page](https://itensor.discourse.group/c/dmrg-and-numerical-methods/8.md) [Next page](https://itensor.discourse.group/c/dmrg-and-numerical-methods/8.md?page=2)