Ensemble Sorting
A guide to sorting ensembles.
Table of contents
Sorting Conformational Ensembles with CREGEN
One of the core procedures of CREST, the sorting of ensembles based on the energy, rotational constants, and Cartesian RMSDs, is refered to as CREGEN and can be invoked as a standalone procedure. CREST is able to sort basically any ensemble file that satisfies the input specifications as given in the Input Formats section.
Assume in the following, that you are working with an ensemble for the alanineglycine from Example 1. As you remember, in the previous example the ensemble was generated with an implicit solvation potential for water. Assume that this was an error and you wanted the ensemble in the gasphase. After refining (optimizing) the GBSA(water) structure for the gasphase and stitching them together in a new ensemble file (here: input-ensemble.xyz), or by using the --mdopt function (Example: Ensemble Optimization), you might be left with an unorderd collection of structures as in the figure below.
You can see that for this ensemble we have 20 structures in total but they are not in an ascending order with regards to their relative energies. Furthermore, some of the structures seem to be degenerate. The question is now: How to sort this ensemble?
The answer is given here:
crest input.tomlcrest struc.xyz --cregen input-ensemble.xyz--cregen command activates the ensemble sorting. The ensemble file (input-ensemble.xyz) is passed as an argument to --cregen. A CREST input structure (struc.xyz) has to be provided to get a reference topology. In the TOML variant all settings are provided in input.toml (CREST >= 3.0). Note that the ensemble must satisfy the format requirements from the Input Formats section.
# This is a CREST input file
input = "struc.xyz"
input_ensemble = "input-ensemble.xyz"
runtype = "cregen"
[cregen]
ewin = 6.0
rthr = 0.125
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C 2.3283860594 -0.1603797181 -0.5381628317
O 1.9699036173 -0.0583259706 -1.6802701315
N -3.4050403026 0.0760327048 0.6837309207
O -0.8369145213 0.5071984003 1.6716396074
O 3.5621572160 0.1242731114 -0.1242140857
H -3.2612986989 1.0076066630 1.0641707334
H -3.4871625446 -0.5522849445 1.4766240918
C -2.1801799929 0.6371343992 -1.3417513822
H -2.3761401991 -1.3226531433 -0.4462734791
H 0.0040226776 -1.2477629483 -0.7913193868
H 1.8808031495 -1.5751066433 0.9943328916
H 1.4216248785 0.0924550492 1.3958684288
H 4.1116194963 0.4338033648 -0.8624421477
H -2.0334814450 1.6655273521 -1.0188928033
H -1.3598951235 0.3569423595 -1.9977727300
H -3.1159159361 0.5775768771 -1.8901876354
20
-33.85745642
C -2.0988939428 -0.3515316703 0.3956355473
C -0.6525021938 -0.0681851959 -0.0250169087
N 0.0273607375 0.6810559930 0.8576667432
C 1.3924965286 1.0795352388 0.6113595503
C 2.1051718387 0.0509305569 -0.2660187085
O 2.6933981539 0.3137618785 -1.2758180775
N -2.5531835473 0.6868312011 1.3152914904
O -0.1509506445 -0.5112485699 -1.0386222266
O 2.1306918400 -1.1512380053 0.3184101027
H -3.4364368602 0.4166015357 1.7346641733
H -2.7174785359 1.5476388311 0.8003316400
C -2.9770960088 -0.5331884563 -0.8440530876
H -2.0690674730 -1.2941385086 0.9578518687
H -0.4732617781 1.0300777126 1.6624708963
H 1.9269438191 1.1375842041 1.5629900806
H 1.4421726503 2.0486199370 0.1007061846
H 2.5370353624 -1.7973258590 -0.2791038133
H -3.9705632606 -0.8795620540 -0.5657636314
H -3.0678403850 0.4110056236 -1.3789395716
H -2.5181332125 -1.2580092257 -1.5097669730
20
-33.85697346
C -2.1912988995 -0.1920605749 0.0425876710
C -0.6753278881 -0.3497323028 0.1580770453
N -0.0725303394 0.7049824376 0.7299762529
C 1.3521541070 0.7253197811 0.9560797243
C 2.0748742052 -0.0979396174 -0.1088631629
O 2.8750334655 -0.9574072859 0.1268227471
N -2.6818225172 0.6282644489 1.1460914484
O -0.0694265717 -1.3097072931 -0.2735009712
O 1.8213606593 0.3654025886 -1.3368797823
H -3.6552425253 0.8696208055 0.9923565985
H -2.6284000106 0.1125551677 2.0186947548
C -2.4886891704 0.5012384401 -1.2876449852
H -2.6250486555 -1.2052212256 0.0208558686
H -0.6642606015 1.4278890987 1.1143135459
H 1.6123248555 0.3092113035 1.9366971664
H 1.7119601266 1.7551951621 0.8871950359
H 2.2435105008 -0.2063927864 -1.9959931774
H -2.0354077841 -0.0559782124 -2.1025047373
H -3.5627308547 0.5565591461 -1.4541410682
H -2.0816445617 1.5091870931 -1.2774730984
20
-33.85973630
C -2.2035273954 0.4764356756 -0.0528000196
C -0.9601767950 -0.4156917332 -0.1698673386
N 0.0819571014 -0.0380065052 0.6109338411
C 1.3635567697 -0.6882702967 0.5192578239
C 2.4622635954 0.3244798665 0.2810936886
O 2.2998863676 1.5147739213 0.2841238852
N -3.1652841554 0.2119700129 -1.1055815296
O -0.9296080285 -1.4057355065 -0.8710872872
O 3.6457176298 -0.2576207261 0.0924030450
H -2.7505962819 0.4037851105 -2.0119394114
H -3.3986491524 -0.7772289940 -1.0968031719
C -2.8655367398 0.2359659486 1.3065896159
H -1.8973662553 1.5266892258 -0.1322119468
H 0.0818572995 0.8775496525 1.0356983279
H 1.3235906572 -1.4047330536 -0.3080203765
H 1.5988784486 -1.2429356884 1.4365622915
H 4.3385712160 0.4071605668 -0.0518143258
H -3.7787293440 0.8213854045 1.3655907293
H -3.1213460876 -0.8159215971 1.4119552049
H -2.2031173676 0.5199089365 2.1197314400
20
-33.85688151
C -2.1193213890 -0.4049908582 0.3584665200
C -0.6629928971 -0.2384092609 -0.0742660577
N -0.1120448442 0.9169005855 0.3296261794
C 1.2274406774 1.2878537714 -0.0575509173
C 2.0863812737 0.0451682677 -0.2868160092
O 2.7315352926 -0.1605345245 -1.2744689696
N -2.3565880832 0.3504418206 1.5842110922
O -0.0675989494 -1.0619487398 -0.7396660608
O 2.1551378656 -0.7084769766 0.8162330042
H -1.8737981326 -0.0848367221 2.3636891938
H -3.3477425846 0.3564688525 1.8009962525
C -3.0063249291 0.1399385168 -0.7622363826
H -2.3063500497 -1.4863433014 0.4642758731
H -0.6779877993 1.5345461436 0.8934950712
H 1.6872304970 1.8712064698 0.7442203032
H 1.2280302470 1.8719121412 -0.9854231640
H 2.6569873014 -1.5163427495 0.6287223931
H -4.0550223023 -0.0463788985 -0.5394243173
H -2.8554710517 1.2118457487 -0.8648269952
H -2.7551806194 -0.3467135179 -1.7004441213
This program call will produce 3 files that are of main interest:
<input-file-name>.xyz.sorted, the sorted input ensemble with an ascending energy order of structures and all duplicates removed.crest_ensemble.xyz, the file containing all unique conformers (i.e., rotamers sorted out).crest.energies, a plaintext file containing the energies for all structures increst_ensemble.xyz.
Taking a look at the output tab above reveals several things for our Ala-Gly example. First, a summary of sorting thresholds is provided. The most important point here is the energy window, which is by default set to 6 kcal/mol. All 20 structures of our ensemble are within this threshold and are analyzed. Two of these structures were duplicates of the same minimum and are removed. Three other structures are identified as rotamers of conformers 1, 3, and 5, respectively. This means, from the inital 20 structures, 18 conformers and rotamers were written to the input-ensemble.xyz.sorted file, and 15 unique conformers were written to crest_ensemble.xyz. The correct ascending energy order of the latter can again be seen in the figure below.
Tip: Many options for adjusting sorting thresholds are available. See the Keyword Documentation for CREGEN.
Handling Topology in CREGEN
You might have noticed that Ala-Gly in the above example and Example 1 was presented in a neutral state and not as a zwitter ion. The zwitterionic species might have been generated during the MTD sampling since GFNn-xTB as an SQM method allows the free forming and breaking of bonds. However, the ensemble sorting procedure CREGEN by default checks the topology of all structures and removes those that do not match a given reference geometry (struc.xyz in our case).
You can try this by adding the following structure to the ensemble and executing the --cregen command again (see the example commands tab below).
20
-33.818790361116
C -2.11918647262311 0.14895122905994 0.24492450423152
C -0.84613409018542 -0.71545902108420 0.09396449210643
N 0.01110878038306 -0.55798901361218 1.12345941613331
C 1.43207934792106 -0.57482696384029 0.88443865999916
C 1.88833437319196 0.82600233879956 0.32894440663697
O 0.97255546731960 1.64669100387907 0.07336052986943
N -1.68369973202269 1.50506796633395 -0.22286995339128
O -0.63218235961918 -1.39453299525113 -0.89224121288301
O 3.10015236848593 1.00313723593259 0.19263098642795
H -1.80652736708673 1.56915945577230 -1.24134242988971
H -0.62343017334745 1.63550171644745 -0.04221327393975
C -3.27633103833500 -0.36059678289799 -0.59454793916073
H -2.39948198144750 0.21252333754469 1.29833445377503
H -0.26102312521712 0.06347936661626 1.87388430943554
H 1.66922672788030 -1.32346211195377 0.12752055915879
H 1.98470863280314 -0.79708838903490 1.79613752154667
H -2.21430989711448 2.26244905004632 0.22131157546907
H -4.09390756675707 0.35408686586709 -0.60424410399088
H -2.94048851129317 -0.55788329271452 -1.60997845699847
H -3.63943941783663 -1.29716616081078 -0.18197510503603
Note: The zwitterion is only stable at the GFNn-xTB level in combination with GBSA or ALPB. For consistency, structure was optimized with
--gbsa water but the energy in was replaced by a gasphase singlepoint calculation at the GFN2-xTB level (-33.818790361116 Eh). To add the zwitterion to the ensemble execute and then execute as before
cat zwitterion.xyz >> input-ensemble.xyzcrest struc.xyz --cregen input-ensemble.xyz --ewin 30 Note: The
--ewin 30 command was added here to increase the energy window to 30 kcal/mol. This is necessary because the zwitter ion has a much higher energy in the gasphase than the neutral Ala-Gly structures and would be sorted out simply by the energy threshold.You will notice that the zwitterion is sorted out immediately, regardless of the energy threshold in CREGEN. This is due to the mismatching topology/connectivity of the reference struc.xyz and the newly added zwitterion.xyz. If you “reverse” the reference, i.e., by using
crest zwitterion.xyz --cregen input-ensemble.xyz
only the zwitter ion should remain because now it was taken for constructing the reference topology.
There is, however, a way to deal with topology-mismatches if required:
All topology checks in CREGEN can be ignored by adding the --notopo command to the program call.
crest zwitterion.xyz --cregen input-ensemble.xyz --ewin 30 --notopo
With this command any isomer of the reference structure in the ensemble file will be considered, as long as it satisfies the format requiements from the Input Formats section.