Monte Carlo methods are often championed as the ultimate parallel algorithms, as- sociated with the phrase “embarrassingly parallel”. Given the evolution of modern computational architectures towards many-core architectures with slower clock speeds, Monte Carlo will continue to play a growing role in the numerical simulation of physics at the boundaries of our computational capabilities. Interacting walker Monte Carlo methods, can be more difficult to parallelize effectively due to the annihilation step
(a)
(b)
Figure 6.6: A schematic representation of the communication patterns the annihilation step of interacting walker Monte Carlo schemes, where the boxes represent different MPI processes and the ellipsis represents the rest of the processes. In the case of the Clock Hamiltonian (a), a time domain decomposition allows one to restrict communication to only nearest neighbor processes, facilitating simple, constant time communication amenable to the architecture of modern parallel computers. In the more general case (b), a clear partitioning may not be readily achievable, and all processes may need to communicate with all other processes, creating a bottleneck.
where communication of walkers is unavoidable.
In contrast to the most general interacting walker algorithm, which may require heavy communication between all processes, the FCIQMC method applied to the Clock Hamiltonian may take advantage of time-locality to create an efficient parallel- in-time algorithm using the standard method of domain decomposition in time. Using this construction, only processes containing adjacent times need to communicate their child walkers, which may be done simultaneously in a time that is constant for the number of processes involved. This remains true so long as the number of time steps under consideration is larger than the number of processes in use, which is typically
0 10 20 30 40 50 60 70 Number of cores 0 2 4 6 8 10 12 14 Parallel speedup Speedup 0.0 0.2 0.4 0.6 0.8 1.0 Parallel Efficiency Efficiency
Figure 6.7: A scaling study of our method implementation with a fixed total problem size (strong
scaling), showing parallel efficiencies and speedups. The simulation consisted of11qubits with128
time points generated by consecutive local rotations withθ ≈ 0.098. The simulation maintained
on average 106walkers in each simulation-time step and the wall clock time was measured to the
point of an equivalent number of statistical samples.
the case. In the case that the number of processes is much greater than the number of timesteps, this scheme may still be used by blocking multiple processors to each time, and utilizing an all-to-all communication pattern within each block only. The difference between these two communication patterns is highlighted in Fig. 6.6.
To demonstrate the scaling properties of this approach, we consider the scaling as a function of the number of processors for fixed total problem size, or strong scaling, with our implementation. This benchmarking is done on a standard Linux cluster composed of AMD Opteron 6376 processors. The parallel speedup with respect to sin- gle core time as a function of the number of processors is given in Fig 6.7. Here, we see that we are able to combine the parallelism of Monte Carlo with the locality of time-decomposition to achieve practical parallel efficiencies of over 95% with 2 proces- sors and 70% with 8 using a simple MPI implementation on a commodity cluster.
6.7 Conclusions
In this work we reviewed the mapping between unitary dynamics and ground state eigenvalue problems. We then showed how the FCIQMC method, a technique orig- inally designed to ameliorate the fermionic sign problem for ground state electronic systems, could be applied to quantum dynamics problems as a direct result of this mapping. This establishes a potential research direction for explicit connections be- tween the fermionic and dynamical sign problems that plague quantum Monte Carlo simulations, and provides a pathway for the transfer of tools between the two do- mains.
The numerical consequences of the dynamical sign problem in this context were studied using a few basic quantum circuits. It was found that even local rotations can exhibit a severe sign problem depending on the form of the rotation and how different it is from a quasi-classical operation. We then introduced a general method analogous to the interaction picture in dynamics or natural orbitals in the study of eigenstates that uses basis rotations to mitigate the difficulty of the problem. The costs and ben- efits of different types of rotations require further research, however we showed that even local rotations can have a significant benefit for non-trivial circuits. Finally, we discussed the structure of the problem in the context of parallel-in-time dynamics, and showed high parallel efficiencies with only a basic MPI implementation on a com- modity cluster.
Overall, we believe this is a promising new method for the simulation of quantum dynamics. It clarifies the bridge between dynamics and ground state problems and is capable of effectively utilizing parallel computing architectures. While we have only demonstrated it for quantum circuits, we believe it will be generally useful for the
Entia non sunt multiplicanda praeter necessitatem. - More things should not be used than are necessary.
William of Ockham
7
Compact wavefunctions from compressed
imaginary time evolution∗
Abstract
Simulation of quantum systems promises to deliver physical and chemical predictions for the frontiers of technology. In this work, we introduce a general and efficient black box method for many-body quantum systems using technology from compressed sens- ing to find compact wavefunctions without detailed knowledge of the system. No
∗Jarrod R McClean and Al´an Aspuru-Guzik. Compact wavefunctions from compressed imaginary time evolution. arXiv preprint arXiv:1409.7358 [physics.chem-ph], 2014.
knowledge is assumed in the structure of the problem other than correct particle statistics. As an application, we use this technique to compute ground state electronic wavefunctions of hydrogen fluoride and recover 98% of the basis set correlation energy or equivalently 99.996% of the total energy with 50 configurations out of a possible 107.
7.1 Introduction
The prediction of chemical, physical, and material properties from first principles has long been the goal of computational scientists. The Schr¨odinger equation contains the required information for this task, however its exact solution remains intractable for all but the smallest systems, due to the exponentially growing space in which the so- lutions exist. To make progress in prediction, many approximate schemes have been developed over the years that treat the problem in some small part of this exponen- tial space. Some of the more popular methods in both chemistry and physics include Hartree-Fock, approximate density functional theory, valence bond theory, perturba- tion theory, coupled cluster methods, multi-configurational methods, and more re- cently density matrix renormalization group [13,17,45,84,102,107,173,192,217,
249].
These methods have been successful in a wide array of problems due largely to the intricate physics they compactly encode. For example, methods which are essentially exact and scale only polynomial with the size of the system have been developed for one-dimensional gapped quantum systems [134]. However such structure is not always easy to identify or even present as the size and complexity of the systems grow. For example, some biologically important transition metal compounds as well as metal
clusters lack obvious structure, and remain intractable with current methods [129,
169,271].
The field of compressed sensing exploits a general type of structure, namely sim- plicity or sparsity, which has been empirically observed and is adaptive to the prob- lem at hand. Recent developments in compressed sensing have revived the notion that Occam’s razor is at work in physical systems. That is, the simplest feasible solution is often the correct one. Compressed sensing techniques have had success in quantum simulation in the context of localized wavefunctions [190] and vibrational dynamics of quantum systems [46,257], but little has been done to exploit the possibilities for many-body eigenstates, which are critically important in the analysis and study of physical systems.
In this letter, we concisely describe a new methodology for finding compact ground state eigenfunctions for quantum systems. It is a Multicomponent Adaptive Greedy Iterative Compression (MAGIC) scheme. This method is general in that it is not re- stricted to a specific ansatz or type of quantum system. It operates by expanding the wavefunction with imaginary time evolution, while greedily compressing it with or- thogonal matching pursuit [232]. Matching pursuit and its variants are greedy algo- rithms in the standard sense, that is, at each step they select a new optimal compo- nent without regard to the consequences this may have on future steps. As an exam- ple application, we choose the simplest possible ansatze for quantum chemistry, sums of non-orthogonal determinants, and demonstrate that extremely accurate solutions are possible with very compact wavefunctions. This non-orthogonal MAGIC scheme we refer to as NOMAGIC, and apply it electronic wavefunctions in quantum chem- istry.