Journal of Quantitative Spectroscopy & Radiative Transfer 255 (2020) 107228
Contents lists available at ScienceDirect
Journal of Quantitative Spectroscopy & Radiative Transfer
journal homepage: www.elsevier.com/locate/jqsrt
The 2020 release of the ExoMol database: Molecular line lists for
exoplanet and other hot atmospheres
Jonathan Tennyson
a , ∗
, Sergei N. Yurchenko
a
, Ahmed F. Al-Refaie
a
, Victoria H.J. Clark
a
,
Katy L. Chubb
a , b
, Eamon K. Conway
a , i
, Akhil Dewan
a
, Maire N. Gorman
a , c
, Christian Hill
a , d
,
A.E. Lynas-Gray
a , g , h
, Thomas Mellor
a
, Laura K. McKemmish
a , e
, Alec Owens
a
,
Oleg L. Polyansky
a , j
, Mikhail Semenov
a
, Wilfrid Somogyi
a
, Giovanna Tinetti
a
,
Apoorva Upadhyay
a
, Ingo Waldmann
a
, Yixin Wang
a , f
, Samuel Wright
a
, Olga P. Yurchenko
a
a
Department of Physics and Astronomy, University College London, London, WC1E 6BT, UK
b
SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, Netherlands
c
Department of Physics, Aberystwyth University, Ceredigion, UK, SY23 3BZ, UK
d
Nuclear Data Section, International Atomic Energy Agency, Vienna A-1400, Austria
e
School of Chemistry, University of New South Wales, Sydney 2052, Australia
f
Nankai University, 94 Weijin Road, Tianjin, China
g
Department of Physics, University of Oxford, England, United Kingdom
h
Department of Physics and Astronomy, University of the Western Cape, South Africa
i
Atomic and Molecular Physics Division, Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA USA
j
Institute of Applied Physics, Russian Academy of Sciences, Ul’yanov Street 46, Nizhny Novgorod, 603950, Russia
a r t i c l e i n f o
Article history:
Received 10 June 2020
Revised 21 July 2020
Accepted 22 July 2020
Available online 24 July 2020
Keywords:
Infrared
Visible
Einstein A coefficients
Transition frequencies
Partition functions
Cooling functions
Lifetimes
Cross sections
k Coefficients
Landé g -factors
pressure broadening
a b s t r a c t
The ExoMol database ( www.exomol.com ) provides molecular data for spectroscopic studies of hot atmo-
spheres. While the data are intended for studies of exoplanets and other astronomical bodies, the dataset
is widely applicable. The basic form of the database is extensive line lists; these are supplemented with
partition functions, state lifetimes, cooling functions, Landé g-factors, temperature-dependent cross sec-
tions, opacities, pressure broadening parameters, k -coefficients and dipoles. This paper presents the latest
release of the database which has been expanded to consider 80 molecules and 190 isotopologues totaling
over 700 billion transitions. While the spectroscopic data are concentrated at infrared and visible wave-
lengths, ultraviolet transitions are being increasingly considered in response to requests from observers.
The core of the database comes from the ExoMol project which primarily uses theoretical methods, al-
beit usually fine-tuned to reproduce laboratory spectra, to generate very extensive line lists for studies of
hot bodies. The data have recently been supplemented by line lists derived from direct laboratory obser-
vations, albeit usually with the use of ab initio transition intensities. A major push in the new release is
towards accurate characterisation of transition frequencies for use in high resolution studies of exoplanets
and other bodies.
©2020 Elsevier Ltd. All rights reserved.
1. Introduction
The ExoMol project was started in 2011 [1] with the purpose of
providing molecular line lists for studies of exoplanets and other
(hot) atmospheres. Besides data demands for exoplanets [2] , other
hot astronomical bodies with significant molecular content in their
atmospheres include cool stars [3] and brown dwarfs [3,4] . On
Earth, similar spectroscopic data are required to study flames [5,6] ,
∗
Corresponding author.
E-mail address: j.tennyson@ucl.ac.uk (J. Tennyson).
discharge plasmas [7] , explosions [8] and the gases emitted from
smoke stacks [9] . In addition, bodies in non-local thermodynamic
equilibrium (non-LTE) such as comets and masers give observable
emissions from highly excited states [10–12] .
ExoMol data are proving very popular among the exoplanet
atmospheric modellers owing to their extensive coverage of the
molecular species and their completeness as a function of both
frequency and, very importantly, temperature. ExoMol data have
been incorporated into many radiative transfer and retrieval codes
including Tau-REx [13–15] , Phoenix [16] , NEMESIS [17] , CHIMERA
[18] , HELIOS [19] and HELIOS-r2 [20] , ATMO [21–23] , ARCiS [24] ,
https://doi.org/10.1016/j.jqsrt.2020.107228
0022-4073/© 2020 Elsevier Ltd. All rights reserved.
2 J. Tennyson, S.N. Yurchenko and A.F. Al-Refaie et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 255 (2020) 107228
ARTES [25] , HyDRA [26] , GENESIS [27] , petitRADTRANS [28] , PLA-
TON [29] , VSTAR [30,31] , BART [32] and Pyrat Bay [33] . In addition
the ExoMol database has been used to support a variety of other
studies. Examples including the analysis of gaseous ammonia in
Jupiter [34] , tentative detection of H
2
S in Uranus [35] , detection
of CaO in meteors [36] and SiO in B[e] Supergiants [37] , Late-type
stars [38] or circumstellar environment [39] . Detections in the
atmospheres of exoplanets are discussed below. More diverse ap-
plications include modelling laser analysis of Solar System objects
[40,41] , isotope abundance quantification in stars [42] , search for
molecules highly sensitive to the proton-to-electron mass ratio
variation [43,44] , models of molecular steering [45,46] , design of
THz lasers [47] , design of dark matter detection schemes [48] and
the design of novel propulsion systems [49] .
A number of diatomic molecules for which ExoMol provides ex-
tensive hot line lists have been detected in sunspots. These include
SiO [50] , SiH [51] , SH [52] , BeH [53] , VO [54] , as well as MgH, MgO,
TiO, C
2
and CaH which have all long been observed in sunspots
[55] . In addition the spectrum of water is well-known in sunspots
[56–60] ; studies of water spectra in sunspots should be aided by
the new POKAZATEL water line list [61] which is designed for stud-
ies at temperatures above 30 0 0 K such as those encountered in
solar umbra and penumbra.
ExoMol line lists have been published as a series in the jour-
nal Monthly Notices of the Royal Astronomical Society (see below)
and a first data release in 2016 [62] , henceforth ExoMol2016, pro-
vided full documentation for the ExoMol database which can be
found at www.exomol.com . The basic form of the database is ex-
tensive line lists; these are supplemented with partition functions,
state lifetimes, cooling functions, Landé g-factors, temperature-
dependent cross sections, opacities, pressure broadening param-
eters, k -coefficients and dipoles. The ExoMol website also of-
fers an extensive bibliography database on research literature on
molecules relevant for ExoMol applications. So far the bibliography
database contains 6709 sources.
Since ExoMol started there have been major changes in exo-
planet science which have driven further expansion and develop-
ment of the ExoMol database. The first is the discovery of rocky
exoplanets orbiting so near to their host star that their surfaces
are likely to be molten. The atmospheres of these planets are
thought to contain a variety of species not considered previously
[63–66] . Secondly higher temperature planets have been observed
than anyone anticipated, the poster child for hot exoplanets is Kelt-
9b which is thought to reach temperatures of 40 0 0 K [67] , hotter
than most of the stars in our Galaxy! This has led to the need to
construct line list for key species over an extended temperature
range. Third is the development of Doppler-shift high resolution
spectroscopy of exoplanets [68,69] which has proved a powerful
tool for detecting molecules but fails in the absence of highly ac-
curate molecular line lists [70] . These developments have led us to
both expand the range of molecules included in the database and
to begin a systematic attempt to improve the accuracy of the line
positions for the line lists contained in the database. Progress on
both of these objectives is described below as well as work on ex-
tending the coverage of the database into the ultraviolet.
Of course ExoMol is not the only database providing spectro-
scopic data for atmospheric studies. For the Earth’s atmosphere,
databases HITRAN [71–73] and GEISA [74,75] provide comprehen-
sive and validated datasets for approximately 50 key molecules.
However, these databases are designed for studies at room tem-
perature and below and do not contain the necessary data for ade-
quately calculating radiative transport in hot bodies. The HITEMP
database was constructed to extend HITRAN to higher temper-
atures. The latest, 2010, release [76] only contained data on 5
molecules (OH, NO, CO, H
2
O and CO
2
) and improved hot line
lists are available for all these molecules; an update to HITEMP
is currently in progress [77–79] . Other relevant databases include
TheoReTS [80] , which contains hot line lists for 8 polyatomic
molecules, and Kurucz’s compilation of data, which is very com-
plete for atomic sources but contains data only on about ten di-
atomic molecules [81] all of which are covered by ExoMol, usu-
ally to higher accuracy. The MoLLIST data base of Bernath and co-
workers [82] contains empirically derived line lists for about 20
diatomic species. These line lists have recently been incorporated
into the ExoMol database [83] , see Table 2 below. Databases of hot
molecular spectra for other specialised applications include those
for combustion [6,84] and studies of laser-induced plasmas [85] .
ExoMol line lists together with data from other sources will serve
as the main data source for exoplanet studies planned for the up-
coming space missions Ariel and JWST.
The methodology [65,86–91] and software [92,93] developed
and used by the ExoMol project has been extensively documented
elsewhere. Here we will only consider those aspects which im-
pinge on the new ExoMol2020 release.
2. Database coverage
The ExoMol project aims at complete coverage of the spec-
troscopic properties of molecules which are deemed to be im-
portant in hot astrophysical environments. Coverage concerns (a)
the molecular species considered, including isotopologues; (b) the
spectroscopic and thermodynamic properties considered; (c) the
frequency range considered and (d) the upper temperature range
for which the data are reasonably complete. Both the required
temperature and frequency range completeness are to some extent
a judgement on what is required for astronomical and other stud-
ies.
Unlike ExoMol2016, the ExoMol2020 database essentially pro-
vides a complete dataset for modelling hot atmospheres. While
new species are still being added, commonly at the request of
users, the database now contains molecular data for 80 molecules
and covers the key molecules thought to be important for ex-
oplanetary studies. In many cases there is more than one line
list for a given isotopologue. For this reason it is our practice to
name each line list, including (unnamed) ones we have imported
from other sources. In the case of multiple line lists the ExoMol
website provides a recommended line list for the given species.
This recommendation is for studies of hot atmospheres; other
available line lists may be more suitable for room temperature
studies.
For ease of understanding we have divided the description of
line lists provided into three tables. Tables 1 and 2 summarise the
molecules for which data are provided by ExoMol and give the
characteristics of the line list in each case. Table 1 details line lists
which have been formally published as part of the ExoMol project
while Table 2 shows line lists imported from other sources which
have been recast in the ExoMol format (see Wang et al [83] for
example); these line lists are fully integrated into the database.
Table 3 lists the main extra line lists available through the ExoMol
website; this table is not comprehensive but the other line lists
available on the ExoMol website and not listed in Table 1–3 are
largely of historic interest only.
3. Individual line lists
An overview of the line lists in the ExoMol database is given in
Fig. 1 .
One general issue is that Medvedev and co-workers
[165,186] identified a numerical problem with the intensities
of high overtone transitions computed with the standard compi-
lation of the diatomic vibration-rotation program Level [187] . Our
diatomic line lists computed with Level or Duo [188] have been
J. Tennyson, S.N. Yurchenko and A.F. Al-Refaie et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 255 (2020) 107228 3
Table 1
Datasets created by the ExoMol project and included in the ExoMol database.
Paper Molecule N
iso
T
max
N
elec
N
lines
a
DSName Reference
I BeH 1 2000 1 16 400 Yadin Yadin et al. [94]
I MgH 3 2000 1 10 354 Yadin ✔ Yadin et al. [94]
I CaH 1 2000 1 15 278 Yadin ✔ Yadin et al. [94]
II SiO 5 9000 1 254 675 EJBT ✔ Barton et al. [95]
III HCN/HNC 1
a
4000 1 399 000 000 Harris ✔ Barber et al. [96]
IV CH
4
1 2000 1 34 153 806 005 YT34to10 ✔ Yurchenko and Tennyson [97] , Yurchenko et al. [98]
V NaCl 2 3000 1 702 271 Barton ✔ Barton et al. [99]
V KCl 4 3000 1 1 326 765 Barton ✔ Barton et al. [99]
VI PN 2 5000 1
142 512 YYLT ✔ Yorke et al. [100]
VII PH
3
1 1500 1 16 803 703 395 SAlTY ✔ Sousa-Silva et al. [101]
VIII H
2
CO 1 1500 1 10 000 000 000 AYTY ✔ Al-Refaie et al. [102]
IX AlO 4 8000 3 4 945 580 ATP ✔ Patrascu et al. [103]
X NaH 2 7000 2 79 898 Rivlin ✔ Rivlin et al. [104]
XI HNO
3
1 500 1 6 722 136 109 AlJS ✔ Pavlyuchko et al. [105]
XII CS 8 3000 1 548 312 JnK ✔ Paulose et al. [106]
XIII CaO 1 5000 5 21 279 299 VBATHY ✔ Yurchenko et al. [107]
XIV SO
2
1 2000 1 1 300 000 000 ExoAmes ✔ Underwood et al. [108]
XV H
2
O
2
1 1250 1 20 000 000 000 APTY ✔ Al-Refaie et al. [109]
XVI H
2
S 1 2000 1 115 530 3730 AYT2 ✔ Azzam et al. [110]
XVII SO
3
1 800 1 21 000 000 000 UYT2 ✔ Underwood et al. [111]
XVIII VO 1 2000 13 277 131 624 VOMYT ✔ McKemmish et al. [112]
XIX H
2
17,18
O 2 3000 1 519 461 789 HotWat78 ✔ Polyansky et al. [113]
XX H
+
3
1
b
3000 1 11 500 000 000 MiZATeP ✔ Mizus et al. [114]
XXI NO 6 5000 2 2 281 042 NOName ✔ Wong et al. [115]
XXII SiH
4
1 1200 1 62 690 449 078 OY2T ✔ Owens et al. [116]
XXIII PO 1 5000 1 2 096 289 POPS ✔ Prajapat et al. [117]
XXIII PS 1 5000 3 30 394 544 POPS ✔ Prajapat et al. [117]
XXIV SiH 4 5000 3 1 724 841 SiGHTLY
✔ Yurchenko et al. [118]
XXV SiS 12 5000 1 91 715 UCTY ✔ Upadhyay et al. [119]
XXVI NS 6 5000 1 3 479 067 SNaSH ✔ Yurchenko et al. [120]
XXVI HS 6 5000 1 219 463 SNaSH Yurchenko et al. [120]
XXVII C
2
H
4
1 700 1 60 000 000 000 MaYTY ✔ Mant et al. [121]
XXVIII AlH 3 5000 3 40 000 AlHambra ✔ Yurchenko et al. [122]
XXIX CH
3
Cl 2 1200 1 166 279 593 333 OYT ✔ Owens et al. [123]
XXX H
2
16
O 1
c
5000 1 5 745 071 340 POKAZATEL ✔ Polyansky et al. [61]
XXXI C
2
3 5000 8 6 080 920 8states ✔ Yurchenko et al. [124]
XXXII MgO 5 5000 5 72 833 173 LiTY ✔ Li et al. [125]
XXXIII TiO 5 5000 13 59 000 000 Toto ✔ McKemmish et al. [126]
XXXIV PH 1 4000 2 65 055 LaTY ✔ Langleben
et al. [127]
XXXV NH
3
1
d
1500 1 16 900 000 CoYuTe ✔ Coles et al. [128]
XXXVI SH 2 3000 2 572 145 GYT ✔ Gorman et al. [129]
XXXVII HCCH 1 2000 1 4 347 381 911 aCeTY ✔ Chubb et al. [130]
XXXVIII SiO
2
1 3000 1 32 951 275 437 OYT3 ✔ Owens et al. [131]
XXXIX CO
2
1 3000 1 7 996 570 390 UCL-4000 ✔ Yurchenko et al. [132]
XL H
3
O
+
1 1500 1 2 089 331 073 eXeL ✔ Yurchenko et al. [133]
Paper Number in series published in Mon. Not. R. Astron. Soc.; N
iso
Number of isotopologues considered; T
max
Maximum temperature for which the line
list is complete; N
elec
Number of electronic states considered; N
lines
Number of lines: value is for the main isotope. ✔ indicates line list recommended
for studies of hot atmospheres.
a
The Larner line list for H
13
CN/HN
13
C due to Harris et al. [134] is recommended.
b
The ST line list for H
2
D
+
due to
Sochi and Tennyson [135] is recommended,.
c
The VTT line list for HDO due to Voronin et al. [136] is recommended.
d
There is a room temperature
15
NH
3
line list due to Yurchenko [137] .
Fig. 1. Molecular line lists: red indicates line lists in progress, blue corresponds to the line lists suggested for molecules specific for hot rocky exoplanets and green indicates
line lists which contain data applicable for high resolution.
4 J. Tennyson, S.N. Yurchenko and A.F. Al-Refaie et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 255 (2020) 107228
Table 2
Datasets not created as part of the ExoMol project but included in the ExoMol database.
Molecule N
iso
T
max
N
elec
N
lines
DSName Reference Methodology
H
2
1 10000 1 4712 RACPPK ✔ Roueff et al. [138] Ab initio
CH 1 5000 4 52201 MoLLIST ✔ Masseron et al. [139] Empirical
NH 1 5000 1 12150 MoLLIST ✔ Brooke et al. [140,141] , Fernando et al. [142] Empirical
OH 1 5000 2 54,276 MoLLIST ✔ Brooke et al.
[143] Empirical
AlCl 2 5000 1 20245 MoLLIST ✔ Yousefi and Bernath [144] Empirical
AlF 1 5000 1 40490 MoLLIST ✔ Yousefi and Bernath [144] Empirical
OH
+
1 5000 2 12044 MoLLIST ✔ Hodges and Bernath [145] , Hodges et al. [146] Empirical
CaF 1 5000 6 14817 MoLLIST ✔ Hou and Bernath [147] Empirical
MgF 1 5000 3 8136 MoLLIST ✔ Hou and Bernath [148] Empirical
KF 1 5000 2 10572 MoLLIST ✔ Frohman et al.
[149] Empirical
NaF 1 5000 1 7884 MoLLIST ✔ Frohman et al. [149] Empirical
LiCl 1 5000 4 26260 MoLLIST ✔ Bittner and Bernath [150] Empirical
LiF 1 5000 2 10621 MoLLIST ✔ Bittner and Bernath [150] Empirical
MgH 2 5000 1 14179 MoLLIST ✔ GharibNezhad et al. [151] Empirical
TiH
1 5000 3 181080 MoLLIST ✔ Burrows et al. [152] Empirical
CrH 1 5000 2 13824 MoLLIST ✔ Chowdhury et al. [153] Empirical
FeH 1 5000 2 93040 MoLLIST ✔ Wende et al. [154] Empirical
HF 2 5000 1 7956 Coxon-Hajig ✔ Coxon and Hajigeorgiou [155] Empirical
HCl 4 5000
1 2588 HITRAN ✔ Li et al. [156] Empirical
CP 1 5000 2 28752 MoLLIST ✔ Ram et al. [157] Empirical
CN 1 5000 3 195120 MoLLIST ✔ Brooke et al. [158] Empirical
C
2
1 5000 2 47 570 MoLLIST Brooke et al. [159] Empirical
CaH 2 5000 1 6000 MoLLIST ✔ Li et al. [160] , Shayesteh et al. [161] Empirical
N
2
1 10000 4
a
7 182 000 WCCRMT ✔ Western et al. [162] Empirical
SiO 1 5000 3 6 67 814 Kurucz-SiO Kurucz [81] Empirical
ScH 1 5000 6 1 152 827 LYT ✔ Lodi et al. [163] Ab initio
LiH 1 12000 1 18 982 CLT ✔ Coppola et al. [164] Ab initio
LiH
+
1 12000 1 332 CLT ✔ Coppola et al. [164] Ab initio
CO 9 9000 1 752 976 Li2015 ✔ Li et al. [165] Empirical
HeH
+
4 9000 1 1430 ADSJAAM ✔ Amaral et al. [166] Ab initio
HD
+
1 9000 1 10285 ADSJAAM ✔ Amaral et al. [166] Ab initio
HD 1 9000 1 5939 ADSJAAM ✔ Amaral et al. [166] Ab initio
CH
3
F 1 300 1 139 188 215 OYKYT ✔ Owens et al. [167] Ab initio
AsH
3
1 300 1 3 600 000 CYT18 ✔ Coles et al. [168] Ab initio
P
2
H
2
b
2 300 1 10 667 208 951 OY-Trans ✔ Owens and Yurchenko [169] Ab initio
P
2
H
2
b
2 300 1 11 020 092 365 OY-Cis ✔ Owens and Yurchenko [169] Ab initio
PF
3
1 300 1 68 000 000 000 MCYTY ✔ Mant et al. [170] Ab initio
CH
3
1 1500 1 2 058 655 166 AYYJ ✔ Adam et al. [171] Ab initio
BeH 3 5000 2 592308 Darby-Lewis ✔ Darby-Lewis et al. [172] ExoMol
CO
2
13 4000 1 298 323 789 Ames-2016 Huang et al. [173,174] ExoMol-like
SiH
2
1 2000 1 254 061 207 CATS ✔ Clark et al. [175] ExoMol
YO 1 5000 6 3520133 SSYT ✔ Yurchenko et al. [176] Ab initio
N
iso
Number of isotopologues considered; T
max
Maximum temperature for which the line list is complete; N
elec
Number of electronic states considered;
N
lines
Number of lines: value is for the main isotope. ✔ indicates line list recommended for studies of hot atmospheres.
a
The WCCRMT line list
considers triplet states only.
b
There are separate line lists for cis and trans P
2
H
2
.
Table 3
Supplementary datasets available from the website.
Molecule N
iso
T
max
N
elec
N
lines
DSName Reference Methodology
H
+
3
2
a
4000 1 3 070 571 NMT Neale et al. [177] ExoMol
H
2
O 2
b
3000 1 505 806 202 BT2 Barber et al. [178] ExoMol
NH
3
2
c
1500 1 1 138 323 351 BYTe Yurchenko et al. [179] ExoMol
HeH
+
4 10000 1 1 431 Engel Engel et al. [180] Ab initio
HD
+
1 12000 1 10 119 CLT Coppola et al. [164] Ab initio
CO
2
13 300 1 161,944 Zak Zak et al. [181,182,183] ExoMol
CO
2
1 4000 1 628 324 454 CSSD-4000 Tashkun and Perevalov [184] Empirical
H
2
O 1 300 1 WAT_UV296 Conway et at. [185] ExoMol.
N
iso
Number of isotopologues considered; T
max
Maximum temperature for which the line list is complete; N
elec
Number of elec-
tronic states considered; N
lines
Number of lines: value is for the main isotope.
adjusted to remove transitions affected by this issue. Medvedev
et al. [189] recently identified similar issues with triatomic sys-
tems but tests suggest that in practice our triatomic line lists seem
largely unaffected by the problem.
Below we consider some of the line lists presented in the Ex-
oMol database and listed in Tables 1 and 2 . We restrict our dis-
cussion to issues not covered in the ExoMol2016 release or the
original publications. We start by considering the 42 molecules for
which line lists have been created as part of the ExoMol series as
listed in Table 1 .
3.1. Diatomics
3.1.1. AlH, Paper XVIII
There is a new AlHambra line list for AlH [122] . A MARVEL
(measured active rotational-vibrational energy levels [190] ) project
was performed as part of this project meaning that many transi-
tions are predicted with experimental accuracy. The AlHambra line
list has been updated to give the uncertainty in the energy in the
States file which allows users to determine the uncertainty in a
given transition wavenumber.
J. Tennyson, S.N. Yurchenko and A.F. Al-Refaie et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 255 (2020) 107228 5
3.1.2. AlO, Paper IX
No change. The APTY line list was recently used to make a de-
tection of AlO in an exoplanet atmosphere, the ultra-hot Jupiter
exoplanet WASP-33 b [191] and hot Jupiter exoplanet WASP-43 b
[192] .
3.1.3. BeH, Paper I
The BeH “Yadin” line list was one of the first constructed by Ex-
oMol [94] ; however, the line list only included transitions within
the X
2
+
ground electronic state. Recently Darby-Lewis et al.
[172] constructed line lists for BeH, BeD and BeT which consider
both the ground and first excited (A
2
) states. At the same time
Darby-Lewis et al. performed a MARVEL analysis and used their
empirical levels in the fit. The Darby-Lewis line list is therefore
more accurate and more extensive than the Yadin one which it
replaces in the ExoMol database and is therefore recommended.
Yadin is still available on the website though renamed to Yadin-
BeH in order to avoid conflict with the “Yadin” line lists for MgH
and CaH, which remain to be recommended for the IR region.
3.1.4. C
2
, Paper XXXI
There are two new line lists for C
2
in the ExoMol2020 release:
ExoMol 8state [124] and the empirical MoLLIST [159] . The MoLLIST
line list only considers the much-observed Swan band while 8state
covers the 8 band systems which interconnect the 8 lowest elec-
tronic states of C
2
. As 8state uses empirical MARVEL energy levels
[193] where available it should be as accurate as MoLLIST for the
Swan band. Use of 8state is therefore recommended. Since MoL-
LIST line lists are recommended for other molecules and in order
to avoid conflicts with the ’recommended’ flag for 8states, the C
2
MoLLIST line list is now referenced to as MoLLIST-C2 on ExoMol.
An update of the C
2
MARVEL data has just been completed
[194] ; this has been used to improve the 8state energies and hence
transition wavenumbers. This latest version includes uncertainties
in the States file.
3.1.5. CaH, Paper I
The “Yadin” CaH line list only considers transitions within the
X
2
+
ground electronic state [94] . MoLLIST provides a rovibronic
line list for the X
2
+
–X
2
+
, A
2
–X
2
+
, B
2
+
–X
2
+
and
E
2
–X
2
+
systems due to Li et al. [160] , Shayesteh et al.
[161] , Ram et al. [195] . Both line lists are included in the ExoMol
database; work is in progress on creating a single unified line list
which will extend the range of rovibronic transitions considered.
In the meantime both line lists are recommended.
3.1.6. CaO, Paper XIII
No change.
3.1.7. CS, Paper XII
No change. We note that a line list for CS has also recently
been supplied by Hou and Wei [196] and Xing et al. [197] have ex-
tended consideration to rovibronic transitions for the lowest, A
1
–X
1
+
, allowed electronic band.
3.1.8. KCl, Paper V
No change
3.1.9. MgH, Paper I
The ExoMol “Yadin” line list for MgH only considers transitions
within the X
2
+
ground electronic state [94] . MoLLIST provides a
rovibronic line list containing A
2
–X
2
+
and B
2
+
–X
2
+
transitions due to GharibNezhad et al. [151] . Both line lists are in-
cluded in the ExoMol database; work is in progress on creating a
single unified line list which will extend the range of rovibronic
transitions considered. In the meantime both line lists are recom-
mended.
3.1.10. MgO, Paper XXXII
There is a new LiTY line list for MgO [125] .
3.1.11. NaCl, Paper V
No change.
3.1.12. NaH, Paper X
No change.
3.1.13. NO, Paper XXI
There is a new NOName line list for NO [115] . This line list was
constructed using a combination of standard ExoMol and empiri-
cal methodologies, and also included a MARVEL project. Its tran-
sition frequencies should therefore be close to experimental ac-
curacy. NOName has largely been adopted for the new release of
HITEMP [77] .
NOName only covers transitions between levels which lie
within the X
2
ground electronic state of NO. A new line list
which includes ultraviolet rovibronic transitions of NO is currently
being constructed.
3.1.14. PH, Paper XXXIV
There is a new LaTY line list for PH [127] .
3.1.15. PN, Paper VI
No change. An extended line list covering visible and ultra
violet (UV), accompanied by a MARVEL project is currently in
progress.
3.1.16. PO, Paper XXIII
There is a new POPS line list for PO [117] .
3.1.17. PS, Paper XXIII
There is a new POPS line list for PS [117] .
3.1.18. SH, Paper XXXVI
There are two new ExoMol line lists for SH: SNaSH [120] which
only covers transitions within the X
2
ground electronic state of
SH and the newer GYT line list of Gorman et al. [129] . GYT con-
siders both transitions within X
2
and vibronic transitions in the
A
2
+
–X
2
band system. As GYT also improves the accuracy of
the X state transitions, it is recommended for all applications. The
SNaSH SH line list is also renamed to SNaSH-SH in order to avoid
a conflict and retained only for completeness.
3.1.19. SiO, Paper II
The ExoMol SiO line lists only consider transitions within the
X
1
+
ground electronic state [95] ; these line lists have been
widely used including a recent determination of isotopologue ra-
tios in Arcturus [198] . For this release of ExoMol, an empirical and
less accurate SiO line list by Kurucz [81] covering the X–X, A–X and
E–X electronic bands was added. Given the importance of SiO for
lava planets [65] , construction of a comprehensive (accurate and
complete) rovibronic line list for SiO covering both IR and ultravi-
olet would be useful.
3.1.20. SiH, Paper XXIV
There is a new SiGHTLY line list for SiH [118] .
3.1.21. SiS, Paper XXV
There is a new UCTY line list for SiS [119] .
3.1.22. SN, Paper XXVI
There is a new SNaSH line list for SN [120] .
