# Afternoon Session: Quantum Monte Carlo

From ALPS

- Chair: Boris Svistunov and Synge Todo
- Speakers
- Synge Todo:
*Parallel Monte Carlo*- Why large load imbalance for parallel tempering?
- Dependence of CPU time on temperature requires fine-tuning

- Multi-canonical ensemble
- Weight depends on global properties of the system

- Why large load imbalance for parallel tempering?
- Simon Trebst:
*Extended statistical ensembles*- More than one variable? Possible, but no examples where it helps significantly
- Energy/temperature are good choices for thermal phase transitions
- Ising: energy allone suffices, cannot get better than N^2 behaviour

- More than one variable? Possible, but no examples where it helps significantly
- Anatoly Kuklov:
*Flowgram Method in QM* - Matthias Troyer:
*The Sign Problem*- Where does it appear?
- Bosonic systems: no
- Fermions: exchanging fermions leads to sign problems
- Bosons in a gauge field: phases
- Frustrated magnets: exchange leads to sign problem

- Can it be solved?
- Basis-dependent!
- In general: NP-hard & QMA-hard
- Scaling with system size

- Where does it come from?
- Sampling with absolute value of the weights is equivalent to sampling bosons to learn about fermions

- Where does it appear?
- Boris Svistunov

- Synge Todo: