May 2014 - Comp-phys-alps-users - lists.phys.ethz.ch

Inconsistent comment in generate_spline_matrix in the DMFT folder
by Kuangshing Chen 22 Jul '15

22 Jul '15

Hi, The function generate_spline_matrix in fouriertransform.C in the dmft folder shows the comments as follows: // A is the matrix whose inverse defines spline_matrix // // 6 6 // 1 4 1 // 1 4 1 // A = ... // // 1 4 1 // -2 0 0 2 However, the following codes, dense_matrix A = 4*dt/6.*boost::numeric::ublas::identity_matrix<double>(Np1); for (int i=1; i<Np1-1; i++) { A(i,i-1) = dt/6.; A(i,i+1) = dt/6.; } A(0,0) = 1.; A(0, Np1-1) = 1.; A(Np1-1, 0) = -2.*dt/6.; A(Np1-1, 1) = -1.*dt/6.; A(Np1-1, Np1-2) = 1*dt/6.; A(Np1-1, Np1-1) = 2*dt/6.; the red lines will make the matrix looks like // 6 6 // 1 4 1 // 1 4 1 // A = ... // // 1 4 1 // -2 *-1* *1* 2 Is the comment correct or the code correct? Any reference for this algorithm? Thank you, Kuang-Shing Chen

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Exact Diagonalization: Heisenberg Hamiltonian, Boundary Conditions
by Daniel Guterding 18 Jun '14

18 Jun '14

Dear ALPS users, I am trying to use the sparsediag application to solve the standard Heisenberg Hamiltonian for various geometries. So far I could however not figure out which Hamiltonian is actually implemented in the model "spin". Is it J*S1*S2, 2J*S1*S2 or 0.5J*S1*S2 ? Generally it would be good to know where in the code the model Hamiltonians can be found. My second question is about boundary conditions. First I tried the "chain lattice" and varied the boundary conditions with the keyword "boundary" set to "open" or "closed" respectively, which had no effect on the determined eigenvalues. Is there a difference between using "open chain lattice" and "chain lattice" with "boundary" set to "open" ? I presume the reason for my problems is a simple user error, hence some help would be appreciated. Best regards, Daniel

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Unconventional hopping
by vitbka konef 05 Jun '14

05 Jun '14

Hi all, Thanks for the help. I did it, in some way! With best regards.

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Variational hopping parameter
by bondnina 28 May '14

28 May '14

Hi dear Alps users. I wonder is there any possibility to setup different hopping parameters for each site in the Hubbard chain and evaluate such a system using TEBD technic. Thank you for help. -- Yours sincerely, Nina Bondarenko 1024R/EAC56154 9F6C DDF8 6873 5D19 FF9A 600E 46A4 42BE EAC5 6154

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Exploiting degeneracy in Hybridization impurity solver
by backes＠th.physik.uni-frankfurt.de 28 May '14

28 May '14

Dear Alps developers, is it possible to add an option for specifying and exploiting the degeneracy of some orbitals in the Hybridization expansion solver? E.g. for a paramagnetic calculation, where only half of the orbitals need to be measured (the input Hybridization function and interaction parameters are identical for these orbitals). best regards, Steffen Backes

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Numerical precision in large Lanczos problems
by Alex Petrescu 12 May '14

12 May '14

Hello ALPS fellows, I encountered constraint-breaking numerical errors in two-point correlation functions evaluated by ALPS on long eigenvectors [sparsediag in ALPS 2.1]. Correlation functions evaluated are <n(i) n(j)> - <n(i)><n(j)> and <bdag(i) b(j)> in a Bose-Hubbard model. I looked at a *large problem*, 18 site lattice (4.9 x 10^6 states with U(1) and translation sym) and a *small problem*, 12 site lattice (~12300 states with the same symmetries). Large problem results are inconsistent* for some non-negligible parameter subset* with those for the small problem. The inconsistency is worse typically where most eigenvector entries are expected to vanish. <n(i)^2> - <n(i)>^2 can become large negative and the set <n(i)> may not obey the constraint on total boson number; the Green's function <bdag(i) b(j)> can occasionally vanish identically. This does not happen everywhere, but on a large parameter subset. Small problem correlation functions are as expected from alternative calculations. Questions: 1) Would this issue be caused by numerical precision problems when the vector gets large? If so, what are the possible flags to pass to sparsediag? 2) A quick look at sparsediag source shows that value_type is either complex<double> or double. This seems to be changeable to <long double>. Are there any other places you might think of that would allow me to build a long double or (complex<long double>) Hamiltonian? Thank you and regards, Alexandru Petrescu

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dynamical correlation functions
by Stefano Di Sabatino 11 May '14

11 May '14

Dear ALPS users, is it possible to obtain from sparsediag or fulldiag, dynamical correlation functions? In particular I'm interested in green functions to compute spectral functions in terms of the frequency. Thanks, Stefano

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Problem building ALPS from source
by Niraj Ghimire 08 May '14

08 May '14

Dear alps users, I have been trying to build alps from source on Fedora 20. I could get the dependent libraries and tools from the instructions page but I have been having issues trying to build alps using a source package. I unpacked the source package and tried to use cmake from the basic instruction given on "Build ALPS" section but I kept getting errors. Since I am a beginner for linux OS, I would really appreciate if there is sufficient information (step-by-step instructions) on "Build ALPS" section. Thank you, Niraj

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