Volume 1 Chapter 2 Search Tests Part 2

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2.8 Screens

2.8.1 Database Screens

When the database is created each entry is computer-analysed and additional information is derived from the basic information which has been extracted from the original publication.

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:

elemental constitution

eg. bit screen 15 corresponds to Group 5A elements, ie. N, P, As, Sb, Bi.

entry content, mainly crystallographic details

eg. bit screen 90 corresponds to R-factor less than or equal to 0.050.

occurrence of letter-pairs in compound names

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.

occurrence of letter-pairs in authors' names

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.

2D chemical connectivity

eg. bit screen 645 corresponds to a 6-membered ring (open or envelope) with exactly one heteroatom.

The above 682 bit screens are detailed in Appendix 1.

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.

2.8.2 Query Screens

When you compose a search question you can assign screens "manually" using SCREEN instructions.

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. Query Screens Assigned by the User

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:

entry contains one or more uncorrected errors
neutron studies

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 Query Screens Automatically Assigned by QUEST and QUEST3D

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.