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This additional information constitutes a screen record of almost 1600 computer bits corresponding to almost 1600 information items.
Each bit is set to 1 or 0 according to whether or not the information is present or absent (true or false) in the entry.
These bit screens can be categorised in the following groups:
eg. bit screen 15 corresponds to Group 5A elements, ie. N, P, As, Sb, Bi.
eg. bit screen 90 corresponds to R-factor less than or equal to 0.050.
eg. bit screen 174 corresponds to the occurrence of any of the following letter-pairs: AR, DB, FL, HV, KF, MP, OZ, RJ, TT, WD, YN.
eg. bit screen 234 corresponds to the occurrence of any of the following letter-pairs: AP, BS, CV, DY, FB, GE, HH, IK, JN, KQ, LT, MW, NZ, PC, QF, RI, SL, TO, UR, VU, WX, YA, ZD.
eg. bit screen 645 corresponds to a 6-membered ring (open or envelope) with exactly one heteroatom.
A further set of bit screens (approx. 900) are reserved for 3D screens pertaining, thus far, to intramolecular distances and torsion angles. They are discussed further in chapter 7.
Additionally, QUEST and QUEST3D will analyse the tests involved in the search question and set screens automatically.
The net result is that a screen record is set up corresponding to your search question.
Each entry in the database has its screen record as the first record of the entry.
As each database entry is read its screen record is compared against the query screen record and only if the query screen record is satisfied will QUEST or QUEST3D search the database entry further.
This screening process greatly increases the speed of searching, especially for connectivity searching where atom-by-atom bond-by-bond matching is very time-consuming.
Normally a user would assign bit screens only in the range 1-155; the assignment of 2D chemical connectivity screens (249-682) should be undertaken with great care and should be done only by experienced users.
Graphics QUEST3D Procedure
In QUEST3D the SCREEN sub-menu, accessed from the SEARCH menu, is used to assign bit screens.
Ex. Suppose we wish to use the following bit screeens:
Use HELP to decide what SCREENS (either POSITIVE or NEGATIVE) you require, then select either SCREEN or *BTEST to define them.
Please type the numbers of SCREENS : (positive or negative) (Press RETURN if you don't want to specify any SCREENS.)
At this point the instruction document contains: SCREen -34 49
Both screens must be satisfied for a hit to be registered, ie. error-free neutron studies.
Basic QUEST Procedure
In QUEST the screen instruction is input by typing: SCRE -34 49
Suppose we have composed the following instruction document:
T1 *SURN KENNARD T2 *SURN RABINOVICH QUES T2.AND.T1
KENNARD contains the letter-pairs KE EN NN NA AR RD. These yield query screens 222 220 222 238 236 229
RABINOVICH contains the letter-pairs RA AB BI IN NO OV VI IC CH. These yield query screens 226 220 224 237 223 227 222 226 220
Some bit screens are common to both authors' names and the net assignment will be: 220 222 223 224 226 227 229 236 237 238.
Note the order of tests in the search question.
T2 should be tested first since the query screens become more powerful as the length of the text increases.
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Volume 1 Chapter 2 Control Instructions.