[RMCProfile-users] Huge Chi2 with the initial run

[Helen Playford - UKRI STFC]

Hi Arun,

Your example file looks fine to me. We'd expect to get a huge value for the Chi2 when first starting, as all the atoms are on their ideal positions and the agreement with the data is *very* poor! In your example the weighting of the co-ordination constraints are very high so that may also increase the reported Chi2 value (depending on whether you are referring to the total or to the Chi2 for an individual dataset).

It's also possible that there is a problem with the scaling of the data which is making things look even worse. Please feel free to send your files over to me at helen.playford at stfc.ac.uk and I can take a look to check all is in order.

Best wishes,
Helen

-----Original Message-----
From: rmcprofile-users <rmcprofile-users-bounces at shadow.nd.rl.ac.uk> On Behalf Of Ramanathan, Arun
Sent: 25 February 2020 03:51
To: rmcprofile-users at rmcprofile.org
Subject: [RMCProfile-users] Huge Chi2 with the initial run

Hello,
     I have synchrotron PDF data. F(Q) was obtained by calibrating using GSAS II. The initial run with a weight of 0.01 for the X-ray reciprocal block seems to give me a huge Chi2 (0.1222E+5). This doesn't seem reasonable. I was wondering if you might be able with this. Here is the .dat file I am using -


TITLE :: Na2SnO3
MATERIAL :: Na2SnO3
PHASE :: C2c
TEMPERATURE :: RT
PRESSURE :: RP
DATA_NOTE :: First
RMC_NOTE :: tutorial example: not necessarily the best fit!
INVESTIGATOR :: Arun

NUMBER_DENSITY ::   0.070712 Angstrom^(-3)
MAXIMUM_MOVES ::   0.02  0.02  0.02 Angstrom
R_SPACING ::  0.0200 Angstrom
PRINT_PERIOD :: 1000
TIME_LIMIT ::  0.0 MINUTES
SAVE_PERIOD :: 0.0  MINUTES


ATOMS :: Na Sn O


FIXED_COORDINATION_CONSTRAINTS :: 2
  > CSTR1 :: 1 3 1.9 2.9 6 1.0 0.00001
  > CSTR2 :: 2 3 1.9 2.6 6 1.0 0.00001

DISTANCE_WINDOW ::
  > MNDIST :: 3.1 3.1 1.9 3.3 1.9 2.8
  > MXDIST :: 4.0 4.0 2.8 4.0 2.5 4.0

INPUT_CONFIGURATION_FORMAT ::  rmc6f
SAVE_CONFIGURATION_FORMAT  ::  rmc6f


IGNORE_HISTORY_FILE ::

FLAGS ::
  > NO_MOVEOUT
  > NO_SAVE_CONFIGURATIONS
  > NO_RESOLUTION_CONVOLUTION

XRAY_RECIPROCAL_SPACE_DATA ::
  > FILENAME :: Na2PrO3.fq
  > DATA_TYPE :: F(Q)
  > FIT_TYPE :: F(Q)
  > CONVOLVE ::
  > FITTED_SCALE
  > RECIPROCAL_SPACE_FIT :: 100 2850 1
  > RECIPROCAL_SPACE_PARAMETERS :: 100 2850 0.01


END ::

Cheers
Arun

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