Week 2

A summary of useful weblinks:

  1. Email Dr Joan Adler with general questions.
  2. Email Dr Joan Adler with your targil solutions.
  3. Class email list.

On 16/11/16 you must hand in a print (or give a website location) of the completed file sur2016.f (no emails of this file please) and email me your results from running this program before 18:00pm.
If you are working on a windows system and running codes on phelafel you can move files to your desktop for printing either by using the command scp, or placing them on a website on phelafel then just download them. You can start a website with these instructions. Make sure to change the name of the results file!

Help for 2016/7 here .

Important update - as of 12/3/14 all the software you need is on phelafel. Alternatives are: not tx (PGPLOT is not installed on TX and I cannot request it because TX is no longer supposed to be used for running codes). Grad students should use aluf, which accesses your tx files too, and you should have an account also there. Undergraduates can use t2.

We will now start to learn about the different types of Computational Physics:

  • simulation,
  • enumeration,
  • algebraic manipulations
  • solution of equations.

    The links to these webpages on these techniques will be updated throughout the semester as appropriate, when we learn about different applications.

    Some general guidelines for Computational Physics are:

  • ALWAYS compare with exact solutions for limiting cases, to validate your programming and algorithms. Do this with each calculation as a powerful way to find the ``trivial'' mistakes.
  • If possible use more than one approach; in particular each of the four classes of approaches listed above is based on different (implicit) assumptions and so use of more than one is a powerful way to cross-check.
  • There is no intrinsic value in calculating on a computer.
  • There is intrinsic value in understanding the physics of complex systems which usually involves use of computers.
  • Visualization is a very important tool for both research and presentation. This is usually, but not always done today on computers. See the discussion in my lecture notes from the Oporto Summer School, 1998.

    Before we start this week's topic lets download the files we need. We will be doing this many times, so today I will take the time to get it right!
    Those who can run with the instructions, please do so. Those who need help please be patient. Today you need a computer with pgplot installed so you have to work on phelafel/a phclass/goglobal. Open a window and a browser. On your computer point your browser here and download the file and save it to disk. This file is called week2.tar and is a unix archive file.

    1. On goglobal go to step 3. If you are on phelafel, or a phclass go straight to step 4. The labels mymachine and machine then both refer to phelafel/phclass.
    2. If you are on anything except phelafel/phclass or goglobal, then as well as the first window to your local machine, `mymachine' open a second window to your account, `myname' on either aluf/t2 or phelafel, `machine'. (If you do not have one sit next to someone who does or use a phclass.) You do this by the command
      ssh -X machine -l myname Then from the first local window on `mymachine' make the command below where `myname' is your account on staff/phelafel and `machine' is staff or phelafel.
      scp week2.tar myname@machine:
    3. If you are using goglobal on tx, you have to ssh -X to aluf to get access to a browser. (netscape &) Then download the file, if it goes to the Downloads directory move it where you want it with the mv command.
    4. Then everyone does (on a window on `machine')
      tar -xvf week2.tar
      and gets 5 files, 4 related to week2/targil2 and one, .cshrc, that is explained below.
    5. If you are not allowed to open the file, make the commannd chmod a+r week2.tar and then try again.
    6. If you have problems with paths on your account on PHELAFEL or PHCLASS1-5 try to fix them by making sure the file .cshrc is in your main directory, then running the command source .cshrc. This should set up the paths that you need (you only need to do this once.) Alternately write ./ before the file name.
    Now we can continue and do physics!

    As an example of simulation and enumeration we will study random walks, learn how to generate random numbers and make simulations and enumerations. The reference for this is Gould and Tobochnik, Chapter 11 first edition and Chapter 7, second edition, although the new edition does not contain all the relevant material. We will also get aquainted with editing, compiling and running programs in a UNIX environment as well as with graphics software to be used in the course.

    Useful programs for doing this are rwalk1.f and rwalk2.f using cplot.f to graph the results. The program cplot.f uses the PGPLOT graphics library. See the commands page for compilation information. More information about PGPLOT (installed on PHELAFEL and TX) can be found at the pgplot home page and a listing of the different subroutines can be found on the local pgplot subroutines page.

    Why PGPLOT? Its good, free, widely avaliable at the Technion, AND has the same style (subroutines in a library) as the more sophisticated 3D graphics package OpenGL (or its clone mesa), which has become the software underlying modern three dimensional visualization.

    The random walk material from this lecture forms the basis for questions 1 and 2 of the second targil.

    Algebraic manipulation is an important part of Computational Physics. At some lecture after week 2 I will try to fit in the start of learning about one of the best ways of doing this, with the MATHEMATICA package. The study of MATHEMATICA will continue in several stages during the semester. MATHEMATICA is an optional tool for several of the targilim, and will be compulsory (MAPLE is also allowed, but will not be taught as it is not so avaliable) for two questions of the fourth targil, . For a first start in MATHEMATICA look at the Computer Center guide to MATHEMATICA on UNIX machines. Please refer to this when you need help on using files and printers.

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  • For more information about the Computational Physics Group at the

    Technion - Israel Institute of Technology

    see the

    Computational Physics Home Page