CASINO manual - Theory of Condensed Matter

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2 The quantum Monte Carlo method The correlated motion of electrons plays a crucial role in the aggregation of atoms into molecules and solids, in electronic transport properties and in many other important physical phenomena. Ab initio electronic structure calculations, in which the properties of such correlated electron systems are computed from first principles, are a vital tool in modern condensed matter physics and molecular quantum chemistry. Currently the most popular way to include the effects of electron correlation in these calculations is density functional theory (DFT). This method is in principle exact, in reality fast and often very accurate, but does have a certain number of well-known limitations. In particular, with only limited knowledge available concerning the exact mathematical form of the so-called exchange– correlation (XC) functional, the accuracy of the approximate form of the theory is nonuniform and nonuniversal, and there are important classes of materials for which it gives qualitatively wrong answers. An important and complementary alternative for situations where accuracy is paramount is the quantum Monte Carlo (QMC) method, which has many attractive features for probing the electronic structure of real systems. It is an explicitly many-body method, applicable to both finite and periodic systems, which takes electron correlation into account from the outset. It gives consistent, highly accurate results while at the same time exhibiting favourable scaling of computational cost with system size. This is in sharp contrast to the accurate methods used in mainstream quantum chemistry, such as configuration interaction or coupled-cluster theory, which are impractical for anything other than small molecules, and cannot generally be applied to condensed matter problems. The use of QMC was greatly hampered over its first two decades of existence by a combination of insufficient computer power and a lack of available, efficient QMC computer programs general enough to treat a similar range of problems to regular DFT codes. Fast parallel computers are now widespread, and you are now in possession of just such a computer program, which is capable of carrying out QMC calculations for a wide range of interesting chemical and physical problems on a variety of computational hardware. As a bonus, casino is capable of exploiting with essentially perfect parallel efficiency as many processors as you like—a great asset in this era of petascale computing where machines with hundreds of thousands or even millions of cores are beginning to appear. casino has been designed to make the power of the QMC method available to everyone and we hope you enjoy using it. 3 Miscellaneous issues 3.1 Support casino has documentation and examples, etc., but it is a research code, and learning how to use it is a significant task. This is particularly the case if the user does not have relevant experience, such as familiarity with the theory of variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC) calculations and knowledge of DFT or Hartree–Fock (HF) methods for atoms, molecules and solids. We have found that supporting users can take up a large amount of our time and so we have to limit the number of groups that we work with directly. Most users need quite a lot of help and their projects turn into collaborations, but of course we cannot enter into too many projects of this type as our time is limited. We often have people visiting Cambridge to learn about the codes and how to do calculations, and this seems to work well. Beginning in 2006 we also organize annual summer schools at MDT’s Tuscan monastery (see vallico. net/tti/tti.html). The scientific side of these week-long events involves over 20 hours of lectures from the main authors of the code with plenty of hands-on tuition, and people who have attended these events have generally found them to be beneficial. Since June 2013, the best place for having your questions about casino answered is over at the casino discussion forum vallico.net/casino-forum. Unless you have a good reason for thinking some particular individual can help you, this forum should be the first place you ask for help. This supersedes the earlier mailing list casino-users ‘at’ tcm.phy.cam.ac.uk which still exists but is now only used by the casino administrators for announcements of, e.g., events and new versions of the code. 2
3.2 Legal stuff casino is given away for free to any academic or individual wishing to use it for non-profit making purposes (commercial companies should make a specific email enquiry). The software may be downloaded from our public website at vallico.net/casinoqmc/download-casino/. The download procedure involves ticking a checkbox agreeing to certain legal conditions, the practical upshot of which is that users may not redistribute the code, they may not incorporate any part of it into any other program, nor may they modify it in any way whatsoever without prior agreement of the Cambridge group (though this is usually very easy to obtain). You may not use or even retain a copy of casino if you have not recorded your agreement to these conditions. A copy of the agreement can be found in the casino distribution in the file CASINO/doc/academic consent form.pdf or CASINO/doc/commercial consent form.pdf (the latter should be used only in the case of profitmaking companies). 3.3 Getting the latest version of the code New versions of casino are produced on a regular basis (nightly builds for the beta version). People downloading the software for the first time should fill in the form on the following web page: vallico.net/casinoqmc/download-casino/. After filling in the form you will be able to download the standard version of the code immediately, and a request for a casino login ID and password will be triggered. This ID is required to update your copy of casino with later updates. It may also be used to access the casino online discussion forum, and to publish ‘Blog’ or ‘Project’ posts on the casino website. Tickboxes requesting these services, as well as ones for joining the casino mailing list, attending the annual summer school, or for offering your services for development work, are all part of the online registration form. In the initial download, users have the option of downloading either the most recent stable version or the ‘current beta version’. The current beta is a build with the latest changes released as they happen, which may or may not be suitable for production work, but is required for development work. Subsequent updates to the code may be downloaded from the site vallico.net/casinoqmc/ update-casino using your casino login ID and password. Users in the Cambridge TCM group who belong to the Unix group ‘casino’ may additionally copy the latest source from the directory ~casino/PUBLIC/current beta, and access the git repository at ~casino/git/CASINO. If you have been accepted as a developer (see vallico.net/casinoqmc/ how-to-become-a-developer/) then your casino login ID will also give you access to the developer version of the code, which you can download from vallico.net/casinoqmc/download-developer/. The developer version is identical to the standard version in all respects, except that the standard version has ‘obfuscated’ source code which has deliberately been made very difficult to read. The developer version has the full documented source code. Note that there exists a mailing list through which all licensed casino developers (who are listed on the page vallico.net/casinoqmc/things-to-do/) are encouraged to communicate with each other. Ask MDT if you wish to be added to or removed from this list. The casino authors use the git revision control system, and it is possible for external developers to have (read-only) access to our git repository. git-formatted patches may be sent to Mike Towler who will (probably) incorporate them into the master repository. It would be greatly appreciated if you could forward a copy of any article published using the results of casino calculations to us, both for our interest and so that we can add references to the casino web pages. If you wish, you may add blog posts to the casino public website using your casino login ID; this is a good way to publicize your research to the QMC community. 4 Functionality of CASINO The casino program continues to be actively developed and many improvements and revisions are envisaged, but in its present state its capabilities are as follows: • Methods: 3
Page 1 and 2: CASINO User’s Guide Version 2.13
Page 3 and 4: 8.6 GAUSSIAN94/98/03/09 . . . . . .
Page 5 and 6: 29.2 Fourier transformation of CASI
Page 7: 1 Introduction casino is a computer
Page 11 and 12: - Grossman-Mitas DMC-DFT molecular
Page 13 and 14: Your defined setup can then be perm
Page 15 and 16: - : perform compilation (default).
Page 17 and 18: 6 Introductory user’s guide: how
Page 19 and 20: ATOM BASIS TYPE The basis used to r
Page 21 and 22: ENVMC v0.60: Script to extract VMC
Page 23 and 24: Std. err. in the mean DMC energy (a
Page 25 and 26: Jastrow factor from each cycle of t
Page 27 and 28: V(x) Ψ init (x) τ{ t Ψ 0 (x) x T
Page 29 and 30: the energy becomes roughly constant
Page 31 and 32: many cores, time limits etc, which
Page 33 and 34: Set the verbosity level of the mach
Page 35 and 36: 6.7 How to run coupled DFT-DMC mole
Page 37 and 38: unqmcmd --startqmc=M Start the chai
Page 39 and 40: config.out This is the name under w
Page 41 and 42: ‘slater-type’: use Slater-type
Page 43 and 44: CUSTOM SPAIR DEP (Block) This input
Page 45 and 46: initial configurations come from DM
Page 47 and 48: DTDMC (Real) Time step for DMC run
Page 49 and 50: FORCES INFO (Integer) Controls the
Page 51 and 52: nucleus is assumed to lie at the or
Page 53 and 54: MOVIECELLS (Logical) If F then casi
Page 55 and 56: OPT MAXITER (Integer) Largest permi
Page 57 and 58: of G vectors and discards all those
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half a million cores, it may be des
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VM REWEIGHT (Logical) If vm reweigh
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default this is 0 hartree. It shoul
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A very detailed specification of th
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-1 0 0 1 1 1 1 1 1 1 0 2 1 0 1 2 Pa
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cusp conditions; otherwise, only th
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Detailed information about each of
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|k| (outermost). Hence one can spec
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large and the wave function is near
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R(i) in atomic units 0.000000000000
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2. The charge-density Fourier coeff
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c_767: [ 996 ] ... Compressed expan
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valence charges for each atom %%%%%
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1988). This can be found online. An
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1.3813695578425E+00 1.3957621401173
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PWSCF Method: DFT DFT Functional: u
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0.125203784849961E-01 0.13042462855
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3.3000000000000E+00 2.0000000000000
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Potential Number of grid points 200
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9. Clearly, the total number of pos
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EBEST (DMC only) Best estimate of t
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Use G-vector set 1 Number of sets 2
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1000.0 rho_a(G)*rho_b(-G) 16.0 4.12
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total weight must always be include
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The exporter does not currently wor
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8.4.2 Generating gwfn.data files wi
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After successfully running properti
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% mv Test.FChk dna.Fchk run gaussia
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• By default, gaussian uses spin
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Routine Purpose qmc write Writes th
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not the case the defaults can be ch
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8.10 TURBOMOLE Website: www.turbomo
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• crysgen06/09, crystaltoqmc: The
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• quickblock: Simple reblocking u
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• In Movie Settings choose Trajec
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the move rejection probability is h
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This leads to the single-electron d
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In the UNR [19] scheme the modified
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13.7 Evaluating expectation values
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Population-control catastrophes sho
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where u k has the periodicity of th
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Although the constraint equations a
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18 Wave-function updating Consider
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19.2 Evaluating the nonlocal pseudo
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of a unit point charge at r j in ev
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19.4.3 1D Coulomb interaction Coulo
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To investigate whether the extrapol
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correlations, such as might be enco
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where n is the order of the expansi
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terms by definition, and it can be
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which is therefore independent of r
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where the local energy, E L , is E
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Another variant of variance minimiz
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The energy minimization method used
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valid. The first problem, which ari
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• Is emin min energy too high? Th
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It may be activated by setting vmc
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that the number of configuration mo
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• It is possible to use blip orbi
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0.5 0.5 0.0 particle 1 det 1 : 9 or
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The clearup twistav script can be u
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In the infinite system limit, the s
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Note that this has the k −2 diver
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• The effective time step, Eq. (3
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1 2 H He 1.00794 4.00260 3 4 5 6 7
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and the Dirac delta function is δ(
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33.2.2 Atomic densities K eywords:
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The spin-density matrix is a two-by
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33.3.3 Homogeneous and isotropic sy
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33.4 Structure factor and spherical
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33.5 One-body density matrix, two-b
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[ where the approximation ρ (1)T
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The figure shows the localization l
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with F HFT mix = − F P mix = −
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The input keyword to activate the c
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To each walker r j , we assign a li
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37 Magnetic fields and the fixed-ph
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38 Analysis of the performance of C
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the orbitals, α = 2 [11]. The use
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39.2 Implementation basics and perf
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• Use ‘endif’ and ‘enddo’
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• Evidence of testing on serial a
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B Appendix 2: Automatic testing of
Page 225 and 226:
• Delete any eepot.data and densi
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* "Generic" CASINO_ARCHs are intend
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- ‘head -n 1 /etc/redhat-release
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for a single job in an ensemble of
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inary executable. CASINO does not r
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otherwise. The following tags are o
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[2] V.R. Saunders, R. Dovesi, C. Ro
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[50] W. Janke, Statistical Analysis
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CASINO manual - Theory of Condensed Matter

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