Path: utzoo!attcan!uunet!deimos!ux1.cso.uiuc.edu!aries!mcdonald From: mcdonald@aries.scs.uiuc.edu (Doug McDonald) Newsgroups: comp.lang.fortran Subject: Re: complex*32? Message-ID: <1990Feb16.145001.173@ux1.cso.uiuc.edu> Date: 16 Feb 90 14:50:01 GMT References: <9002120228.AA19489@euler.Berkeley.EDU> <20285@bcsaic.UUCP> Sender: news@ux1.cso.uiuc.edu (News) Reply-To: mcdonald@aries.scs.uiuc.edu (Doug McDonald) Organization: School of Chemical Sciences, Univ. of Illinois at Urbana-Champaign Lines: 29 In article <20285@bcsaic.UUCP> carroll@bcsaic.UUCP (Jeff Carroll) writes: >In article khb@chiba.Eng.Sun.COM (Keith Bierman - SPD Advanced Languages) writes: >> >>IEEE double is large enough that quad precision (*32) isn't needed. I >>neglected to be specific in my previous posting. _roughly_ speaking >>ieee dp is good from 10**-308 to 10**+308. >> > > This reminds me of the answers I used to get from a certain >vendor of test equipment based in Palo Alto. When I thought of a >really creative application for one of their boxes, the salesman would >ask, "Why would you want to do THAT???" > > The WHOLE POINT is that some of us think that we *may need* >COMPLEX*32. > I don't know about COMPLEX*32, but REAL*16 is most surely needed - I used to do classical mechanics of molecules. THE differential equations are unstable - they generate chaotic solutions. Errors for any reason, including roundoff, grow exponentially with time (and number of integration steps, as these equations need fixed step size integrators). Thus to double the length of time you get "correct" answers, you must doubel the word length. To get useful results you may NEED REAL*16, and to verify that a computation for which REAL*8 is good enough by back-integration, you need REAL*16. Doug McDonald