On 28 Nov 2009, at 17:19, Fernando Cucchietti wrote:
> Dear Matthias,
>
> Thanks for the prompt response!
> I want to simulate a spin lattice system with longer-than-first neighbor interaction. Although this could lead to some frustration, in practice it does not because the interaction strength decays fast with distance. So, nothing too fancy.
It will probably still give you a sign problem, which is always an exponential problem.
> What Green's function is measured by the applications? Does the looper code measure it? Perhaps I can make do with sigma^plus_i sigma^minus_j...
Again it depends on the model: Do you think of a Heisenberg model or some other interaction?
Matthias
>
> Thanks,
> Fernando
>
> On Nov 28, 2009, at 4:35 PM, Matthias Troyer wrote:
>
>> Dear Fernando Cucchietti,
>>
>> Indeed the ALPS QMC applications do not support these measurements except for the Green's function sigma^plus_i sigma^minus_j in some of the codes. To answer how easy or hard it is to implement these measurements we would need to know which model you want to simulate.
>>
>> Matthias
>>
>> On 28 Nov 2009, at 16:05, Fernando Cucchietti wrote:
>>
>>> Hello all,
>>>
>>> I am learning how to use the ALPS applications. I had reasonable success with DMRG with which I am experienced, however I was trying to study two dimensional systems with QMC and I ran into a problem: The ALPS application complains that it will not measure off-diagonal observables such as sigma_x, even less for two operators like sigma^plus_i sigma^minus_j. It measures without any problems sigma_z and correlations of sigma_z operators.
>>>
>>> Now, my ignorance of QMC is such that I don't know if this is an intrinsic limitation of Quantum Montecarlo algorithms or just of the particular example application distributed with ALPS -- this is my first question. My second question is: if it is possible to measure e.g. sigma_x, could you give me a rough estimate of how difficult could it be to implement it with the libraries?
>>>
>>> Thanks in advance,
>>>
>>> Fernando Cucchietti
>>>
>>>
>>
>