Cases Of CL Research Articles

Implementierung der multizentrischen Basisdokumentation CL-BaDo für den Konsiliarund Liaisondienst: Generierung von Daten für internes Qualitätsmanagement und Kostenkalkulation/ Implementation of the multi-center basis documentation CL-BaDo for conciliar and liaison services: Generating data for internal quality management and cost calculation

In the German DRG system the funding of CL services is not ensured. The documentation of psychiatric comorbidity and CL care delivery is a pre-condition to the development of funding models for CL-services. A task force of several German psychosomatic associations (German College of Psychosomatic Medicine, German Society of Psychosomatic Medicine and Psychotherapy, General Medical Society for Psychotherapy) developed a new documentation form for CL-services (CL-BaDo). The pilot study explored the multicenter implementation of CL-BaDo and the use of the documentation form for quality management and cost calculation. Over a period of at least three months, participating CL-services documented all CL cases consecutively with the CL-BaDo. One site applied full electronic data processing. 2116 CL cases from eight psychosomatic CL-services were analysed. The CL-BaDo is a time-efficient, feasible and acceptable documentation form for CL-service delivery. The full electronic data processing enables networking with a hospital information system to produce higher data quality. The data of CL-BaDo can be used locally for quality management, development of management strategies and communication with consultants, as well as nationwide for health policy questions and research.

SPECIES ASSIGNATION OF LEISHMANIA FROM HUMAN AND CANINE AMERICAN TEGUMENTARY LEISHMANIASIS CASES BY MULTILOCUS ENZYME ELECTROPHORESIS IN NORTH ARGENTINA

Sixteen Leishmania stocks, 15 isolated from patients with cutaneous (CL), mucocutaneous (MCL), or recurrent cutaneous leishmaniasis, plus one from a dog with CL in Salta and Corrientes Provinces, Argentina, were studied by multilocus enzyme electrophoresis. Thirteen of the stocks from humans were grouped in two zymodemes; nine termed as KMS 1, four as KMS 2, and assigned to Leishmania (Viannia) braziliensis. Two additional stocks from CL cases expressed a KMS 4 enzyme profile, corresponding to L. (V.) guyanensis. Although the parasites from the dog were also assigned to L. (V.) braziliensis, its zymodeme, KMS 3, was not expressed in any of the current human isolates. The characterization of Leishmania from a dog was done for the first time in Argentina. The importance of the intraspecific polymorphism in the induction of clinical forms and in the host-reservoir concept is briefly discussed, based on the zymodeme data of isolates from humans and dogs. The presence of L. (V.) guyanensis was confirmed in the country.

Relationship of Genetic Variants in Coding Regions of thee Msx-1 gene to nonsyndromic cleft lip with or without cleft palate patients and controls

Statement of the Problem: Cleft lip and cleft palate (CL/P) are the most frequent congenital craniofacial birth defects in humans, affecting 0.67:1000 live births. The etiology of nonsyndromic cleft lip with or without cleft palate (CL ± CP) is believed to be multifactorial and to result from the interaction of genetic and environmental factors. The Msx-1 gene has been implicated as being a contributing factor in mediating epithelial-mesenchymal interactions during embryologic development of craniofacial and palatal bones. Recently, it was suggested that genetic alterations of the Msx-1 transcription factor may be etiologically important in at least some cases of nonsyndromic CL ± CP. The objective of this study is to characterize genetic variation in coding regions of the Msx-1 gene and to determine if any of the genetic variants are associated with CL ± CP.

Prenatal diagnosis of dysmorphic syndromes by routine fetal ultrasound examination across Europe.

Ultrasound scan in the mid-trimester of pregnancy is now a routine part of prenatal care in most European countries. The objective of this study was to evaluate the prenatal diagnosis of dysmorphic syndromes by fetal ultrasound examination. Data from 20 registries of congenital malformations in 12 European countries were included in the study. There were 2454 cases with congenital heart diseases, 479 of which were recognized syndromes, including 375 chromosomal anomalies and 104 syndromes without chromosomal anomalies. Fifty-one of the 104 were detected prenatally (49.0%). One hundred and ninety-two of 1130 cases with renal anomalies were recognized syndromes, including 128 chromosomal anomalies and 64 syndromes without chromosomal anomalies; 162 of them (84.4%) were diagnosed prenatally, including 109 chromosomal anomalies and 53 non-chromosomal syndromes. Fifty-four of the 250 cases with limb defects were recognized syndromes, including 16 chromosomal syndromes and 38 syndromes without chromosomal anomalies; 21 of these 54 syndromes were diagnosed prenatally (38.9%), including 9 chromosomal syndromes. There were 243 cases of abdominal wall defects including 57 recognizable syndromes, 48 with omphalocele and 9 with gastroschisis; 48 were diagnosed prenatally (84.2%). Twenty-six of the 187 cases with diaphragmatic hernia had recognized syndromes, including 20 chromosomal aberrations and 6 syndromes without chromosomal anomalies. Twenty-two of them (84.6%) were detected prenatally. Sixty-four of 349 cases with intestinal anomalies were recognized syndromes; 24 were diagnosed prenatally (37.5%). There were 553 cases of cleft lip and palate (CL(P)) and 198 of cleft palate (CP) including 74 chromosomal anomalies and 73 recognized non-chromosomal syndromes. Prenatal diagnosis was made in 51 cases of CL(P) (53.7%) and 7 of CP (13.7%). Twenty-two of 188 anencephalic cases were syndromic and all were diagnosed prenatally. Of 290 cases with spina bifida, 18 were recognized syndromes, and of them 17 were diagnosed prenatally. All 11 syndromic encephaloceles were diagnosed prenatally. Around 50% of the recognized syndromes which are associated with major congenital anomalies (cardiac, renal, intestinal, limb defects, abdominal wall defects and oral clefts) can be detected prenatally by the anomaly scan. However the detection rate varies with the type of syndrome and with the different countries' policies of prenatal screening.

The hydration structures of F−and Cl−investigated by ab initio QM/MM molecular dynamics simulations

Combined ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations have been performed to investigate the hydration shell properties of F− and Cl−. The chemically most relevant region, the hydration sphere of the anions, was treated by Born–Oppenheimer ab initio quantum mechanics using D95V+, 6-31+G and D95V++ basis sets for F−, Cl− and water, respectively, while the remaining part was described by classical pair potentials. The QM/MM simulations have predicted average coordination numbers of 4.6±0.2 for F− and 5.6±0.1 for Cl−, in contrast to the corresponding values of 5.8±0.1 and 5.9±0.1 resulting from classical pair potential simulations. Within the first hydration shell of F−, the QM/MM results indicate more flexibility of the hydration complex in which the F−⋯H–O bond appears to be linear. For the case of Cl−, a combination of linear and bridged forms, together with a competition between the solvation of the ion and hydrogen bonding among water molecules, are observed.

The high sensitivity of a PCR-ELISA in the diagnosis of cutaneous and visceral leishmaniasis caused by Leishmania infantum.

In general, the conventional techniques available for the diagnosis of leishmaniasis have relatively low sensitivity. This means that parasite-rich samples (which can usually only be collected by very invasive methods, such as bone-marrow aspiration) must be employed. This problem has not yet been solved even by use of the PCR-based techniques currently available. However, a new PCR-ELISA has been developed for the diagnosis of cutaneous (CL) and visceral leishmaniasis (VL) caused by Leishmania infantum. This assay appears to have sufficient sensitivity to be effective in the diagnosis of VL not only when bone-marrow aspirates are investigated but also when the samples are of peripheral blood. Overall, the ability of the PCR-ELISA to detect Leishmania, in 76 samples (22 of peripheral blood, 36 bone-marrow aspirates and 18 skin samples) from 72 patients living in a endemic region, was better than that of culture or the examination of Giemsa-stained smears. For example, L. infantum kDNA was detected by PCR-ELISA in 15 (83%) of the 18 skin samples from suspected cases of CL, whereas the combined use of several classical techniques only confirmed the presence of amastigotes in five (28%) of these samples. Similarly, only 21 individuals were diagnosed as having VL by conventional techniques whereas 30 were found Leishmania-positive in the PCR-ELISA. The new PCR-ELISA also appears to be a suitable technique for detecting leishmanial kDNA in samples of peripheral blood from cases of L. infantum-HIV co-infection. The assay is more sensitive than the combined use of several conventional techniques in the diagnosis of subclinical VL, probably because those with subclinical infection have relatively low parasitic loads that are generally undetectable using the other techniques.

A Theoretical Study of the Substituent Effects on the P–X (X = H, F, Cl) Bond Dissociation Energies in para- and meta-Substituted Aromatic Phosphines

Abstract The P–X (X = H, F, and Cl) bond dissociation energies (BDEs) for a number of para- and meta-substituted aromatic phosphines were calculated using R(O)MP2/6-311++g(d, 2p)//(U)B3LYP/6-31g(d) method. The results showed the importance of the radical effect as well as the ground effect on the BDEs. For X = H case, because the substituent affected the neutral phosphine to the same extent as it affected the radical, substituent effects on the P–H BDE were small (ρ+ = 0.28 kJ/mol). In comparison, for X = F or Cl case, because the substituent affected the neutral phosphine much more strongly than it affected the radical, substituent effects on the P–F (ρ+ = −3.25 kJ/mol) or P–Cl (ρ+ = −1.31 kJ/mol) BDEs were significant. Therefore, the substituent effects on BDEs of the Z–X bonds in compounds of the general formula 4-YC6H4Z–X varied when X changed, which meant that a recent proposal (J. Am. Chem. Soc., 123, 5518 (2001)) was unfortunately not supported.

Computed energetics for etching of the Si( [formula omitted]) surface by F and Cl atoms

Energetics and limited reaction pathways have been computed using density functional theory with the B3LYP functional for the reaction of F and Cl atoms with the Si(1 0 0) surface. The energetics are in good agreement with the earlier ab initio calculations of Wu and Carter for the bonding of 1–3 F atoms to a surface dimer. The present energetics are also in good agreement with the results of Srivastava et al. using the Stillinger–Weber potential. We find no barrier for adding a third F atom to a monofluorinated dimer. A significant new result is a low energy pathway to SiF 4, which has not been previously characterized. We find that Cl differs from F in having smaller binding energies and much larger non-bonded repulsions. Adding a third Cl to a monochlorinated dimer leads to a barrier of about 0.6 eV (for a symmetry constrained search) and step edge sites are strongly favored in the Cl case, but not in the F case. The energetics obtained for Cl on Si(1 0 0) are in good agreement with results obtained by Halicioglu and coworkers using the Feil potential.

照射細菌芽胞の耐熱性変化

The effect of heat resistance of endospores after gamma-irradiation were investigated for application to decontaminate microorganisms in spices or other dried food materials. D10values of endospores dried on glass fiber filter with additives of 2% peptone + 1% glycerin were obtained to be 1.8 kGy forBacilluspumilus, 1.1 kGy forB. cereus, 2.2 kGy forClostridium botulinumType A and 2.4 kGy forCl. botulinumType E, respectively. Non-irradiated endospores ofB.pumilusand B. cereuswere inactivated with 40 min of heat treatment at 100°C in water. When endospores were irradiated with 5 kGy, these bacteria were inactivated below 20 min of heating at 100°C in water. In the case ofCl. botulinumType A and Type E, non-irradiated endospores were inactivated with 30 min of heat treatment at 100°C in water, and irradiated endospores at 5 kGy were inactivated below 15 min of heating. However, all of these bacteria could not inactivate endospores even after heating more than 2 hours at 100°C in dry condition or 80°C in water, and decreased a little of necessary times for inactivation of heat treatmentafter irradiation at 5 kGy.

Theoretical studies on thiocarbonyl group transfer reactions

The gas-phase thiocarbonyl transfer reactions X− + RC(S)Z ⇌ RC(S)X + Z− have been investigated with X, Z = Cl, Br and R = H, CH3 at the MP2 and G2(+) levels using the MP2/6-311+G** optimized geometries. The thiocarbonyl transfers proceed by a stepwise mechanism through a triple-well potential energy surface involving a tetrahedral intermediate, T−. The strong proclivity toward the stepwise path is caused mainly by the low-lying πCS* level coupled with the high energy σC–Z* orbital. The reaction barriers, ΔG≠, are higher than the corresponding values for the carbonyl transfers, excepting for the X = Z = Cl case at the G2(+) level. The major factors for these elevated barriers, despite the low deformation energies (ΔEdef), are lower proximate σ–σ* charge transfer energies (δΔEσ–σ*(2)) and the relatively high electrostatic repulsion (ΔEes > 0) in the transition state due to the highly polarized structure with a large negative charge on the S atom. The occurrence of a well-defined intermediate, T−, in the reaction coordinate leads to a relatively early transition state with a low degree of bond making and breaking in the transition state.

Serodiagnosis and epidemiology of visceral leishmaniasis in Turkey.

Infantile Mediterranean visceral leishmaniasis (IVL) and anthroponotic cutaneous leishmaniasis (ACL) have long been known to exist in the western and southeastern Turkey, respectively. To further study these and other related diseases, a recombinant antigen (rK39) specific to VL was used in an ELISA for serodiagnosis of selected patients and for screening dog reservoir populations in several endemic sites. Among 24 confirmed VL cases from western Turkey, the rK39 ELISA proved to be more sensitive than a combination of cultivation and microscopy of bone marrow aspirates. The specificity of rK39 for leishmaniasis was demonstrated by its lack of cross-reactivity with sera from other human diseases in the same sites. Interestingly, six of the 83 parasitologically proven ACL cases from southeast Turkey were also rK39 positive. The end point titers of the positive VL and CL cases vary from 10(-2) to 10(-5) and from 10(-2) to 10(-3), respectively. The rK39 ELISA was also used to screen 494 apparently healthy dogs from Urfa in southeast Turkey, Manisa/Alasehir near the Aegean Sea, and Karabuk near the Black Sea. Eighteen rK39-positive cases (3.6%), all from the latter two areas, were found to have varying endpoint titers (10(-2)-10(-4)). The high titers predicted increased severity and frequency of the clinical symptoms (i.e., lymphadenopathy, depilation, skin lesion, weight loss and/or death), which were manifested subsequently in 16 of these 18 cases. In addition, more positive canine cases were diagnosed by the rK39 ELISA preclinically than the procedures to detect parasites postsymptomatically in the lymph node aspirates. The use of the rK39 ELISA as a sensitive tool makes it possible to demonstrate coendemicity of canine and human VL, as expected in the case of IVL. The results also point to the possible presence of additional VL types in western Turkey and cutanovisceral type in the southeast part of this country.

Study of the total and partial fragmentation dynamics of Ar–HCl after uv photodissociation

The uv photolysis of the Ar–HCl cluster is studied applying an exact time-dependent wave packet method in three dimensions, assuming zero-total angular momentum. The photodissociation process is found to occur via two different fragmentation mechanisms, depending on the initial excitation energy of the cluster. One mechanism leads to total dissociation of the complex, producing three fragments, Ar–HCl+hν→H+Ar+Cl. The fragmentation dynamics in this case is governed by resonance states at relatively low energies of the cluster, in which the H atom collides a number of times with Ar and Cl before dissociating. Manifestations of these collisions are found in the final kinetic energy distribution of the photofragments, which is redshifted in the case of the H fragment, and blueshifted in the Ar and Cl cases. The second type of mechanism consists of a fast and direct photodissociation of the hydrogen, leading to a partial fragmentation of Ar–HCl into hot H fragments and bound Ar–Cl radical molecules. This mechanism dominates at higher energies, which are those mostly populated by the wave packet initially prepared in the present calculations. The experimental implications of the results are discussed.

Desorption and diffusion at pulsed-laser-melted surfaces: The case of chlorine on silicon

The kinetics of thermal desorption from an excimer-laser-melted surface, on which diffusion into the bulk competes with desorption, is numerically evaluated and compared with a variety of experimental data for the case of Cl on Si: Auger electron spectroscopy, time-of-flight mass spectrometry, secondary-ion-mass spectrometry, and transient reflectivity. The model calculations involve nonequilibrium thermal diffusion, phase transition, segregation, and first- and second-order desorption kinetics. With the assumption that the pre-exponential factors of the desorption rates do not change on the liquid surface with respect to the solid one, activation desorption energies are found lower by \ensuremath{\approx}0.5 eV for the liquid surface than for the Si(100):Cl solid surface. This difference is of the order of magnitude of the latent heat of melting. The segregation coefficient of Cl at the liquid-solid interface is 0.02 at a recrystallization speed of \ensuremath{\approx}6 m/s. The calculations also bring information on the dynamics of desorption and melting. The desorption rate from the liquid reaches the large value of \ensuremath{\approx}1 ML/ns. However, the surface is depleted from Cl atoms by both desorption and diffusion in a fraction of nanosecond (allowing only \ensuremath{\approx}60% of the Cl to desorb). Multipulse experiments are also calculated. After only three laser pulses, only the level of Cl contamination (not the shape of the Cl depth profile) varies with laser count. The laser cleaning rate follows the laser-pulse count with a logarithmic law. It takes about five laser pulses to decrease the Cl contamination by one order of magnitude.

Ab initio study of the reaction of chlorine atoms with H2S, CH3SH, CH3SCH3 and CS2

The geometries of adducts of Cl with H 2 S, CH 3 SH, CH 3 SCH 3 and CS 2 have been optimised in ab initio MP2(Full)/6-311G** calculations and vibrational frequencies have been calculated. The strength of bonding increases along the series H 2 S, CH 3 SH, CH 3 SCH 3 with the replacement of a hydrogen by CH 3 increasing the binding energy by ca. 25 kJ mol -1 . Transition states for the hydrogen abstraction reactions for Cl with H 2 S and CH 3 SH have been located at the same level of theory. In the case of H 2 S, hydrogen abstraction could make a significant contribution to the reaction. The hydrogen-abstraction reaction is expected to be important in the case of Cl+CH 3 SH.

766 cases of oral cleft in Italy

Epidemiological and genetic variables for oral clefts were analysed for the years 1981-1989 in a case-control study of congenital malformations in the Emilia Romagna, Veneto, and Friuli regions, and in the Trento and Bolzano hospitals. Birth prevalence for all cases of cleft lip with or without cleft palate (CL(P)) was 8.2 per 10,000 births, and that for cleft palate only (CP) was 6.1 per 10,000. Coexisting abnormalities were found in 23% of CL(P) cases and in 43% of CP. No clusters in time or space were detected. For isolated clefts, a predominance of males among CL(P) and of females among CP was found; epilepsy was the only maternal risk factor correlated with clefts, and an association between clefting and consanguinity was found. Empirical recurrence risks were calculated in both isolated CL(P) and CP.

Quadrupole effects of [formula omitted] nuclei on the solid-state magic-angle spinning nuclear magnetic resonance spectra of [formula omitted] nuclei Deviations from first-order theory and implications concerning the sign of the indirect coupling constant

The effect of the interaction between spin- 3 2 and spin- 1 2 nuclei on solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of the latter is studied in cases where deviations from first-order theory are expected. A comparison is made between the exact and first-order perturbation approaches. Both dipolar and indirect (iso- and anisotropic) coupling interactions are considered. Implications regarding 13C, 35,37Cl, 31P, 63,65Cu and 119Sn, 35,37Cl cases are discussed. It is shown that in the latter two cases the sign of the indirect coupling constant J can be derived.

Cleft lip (± cleft palate) in Danish twins, 1970–1990

A classical twin study is one of the best methods to address the open question of the role of genes and environment in the cause of cleft lip with or without cleft palate [CL(P)]. In addition, when twin concordance rates are combined with information about the risk for CL(P) to more remote relatives, they can help to establish the most likely mode of inheritance for CL(P). The present study was based on three nation wide ascertainment sources of CL(P) in twins in Denmark during the period 1970-1990. The Danish surgical files were found suitable for ascertaining twins with non-syndromic CL(P) and a total of 39 pairs was identified through these files. In more than 70% of the cases, the zygosity assignment was based on unlike-sex or an extensive panel of blood, serum, and enzyme types. More mono- and dizygotic twin (CL(P) cases than expected were found, although the difference was not significant. The proband concordance rate for CL(P) was 60% in monozygotic twins and 10% in dizygotic twins; six cases were of unknown zygosity. This finding indicates that genetic factors play a major role in the cause of CL(P) but environmental and/or stochastic factors are probably acting too. When the familial recurrence patterns analyzed by Mitchell and Risch [1992: Am J Hum Genet 51:323-332] are interpreted in light of these new estimates of monozygotic concordance, they provide further evidence for the finding that no single locus can account for more than a six-fold increase in risk to first-degree relatives.

Crystal structure, infrared and 31P nuclear magnetic resonance studies of complexes of tris(2,4,6-trimethoxyphenyl)phosphine with mercury(II) halides

The 31P NMR spectra of 1:1 solutions of tris(2,4,6-trimethoxyphenyl)phosphine (tmpp) and mercury(II) halides HgX2(X = Cl, Br or I) in acetonitrile reveal the presence of the ionic complexes [HgX(tmpp)]+ and [Hg(tmpp)2]2+. The amount of the first of these two species relative to the second increases from X = Cl to I. The solids which crystallize from tmpp–HgX2 solutions in acetonitrile contain mixtures of ionic compounds which contain both of these cationic complexes. The first species was identified in the solid state by the observation of ν(HgX) bands in the far-IR spectra. In the X = Cl case, crystals of the complex [Hg(tmpp)2]2+[Hg2Cl6]2– were isolated from the solid product and the structure of this complex was determined by single-crystal X-ray diffraction [monoclinic, C2/c, a= 32.767(5), b= 11.500(2), c= 23.814(4)A, β= 133.57(1)°, Z= 4 formula units; R= 0.047 for 3461 ‘observed’ reflections]. The cation has C2 symmetry and the mercury atom environment in this ion is dominated by a close association with the phosphorus atoms of the two attached ligands in a quasi-linear P–Hg–P array, but significant deviations from linearity arise, presumably because of contacts with the methoxy oxygen atoms [Hg–P 2.389(5)A, P–Hg–P 166.51(9)°]. The Hg ⋯ O contacts involving the nearest ortho-methoxy oxygen atoms on the three phenyl groups in each ligand are 2.726(9), 2.938(7), 3.044(9)A. The [Hg2Cl6]2– anions have a structure of Ci symmetry in which two mercury atoms with tetrahedral HgCl4 co-ordination are linked by sharing two bridging Cl atoms. The structural parameters for this anion are similar to those found previously for this species in compounds with other cations. The far-IR spectra of the solids obtained from the other mercury halides reveal the presence of [Hg2X6]2– species in these compounds also. For X = I, the product is predominantly [Hgl(tmpp)]2[Hg2I6].

In situ real-time determination of the free-carrier density in doped ZnSe films during molecular beam epitaxial growth

The characteristic blue/green cathodoluminescence (CL) emission observed while analyzing doped ZnSe films using the reflection high energy electron diffraction technique during molecular beam epitaxial growth has been detected ex vacuo and its intensity has been quantified. By employing a CdS-based photoresistor mounted outside the vacuum system it has been determined for the examined case of Cl doped n-type ZnSe/GaAs films that the CL intensity varies linearly as a function of free-carrier density in the films. Specifically, log(CL intensity) was found to be proportional to log(free-electron density) over the room-temperature free-electron density range, 1017–5×1018 cm−3. This work demonstrates that the free-carrier density in doped ZnSe films grown by molecular beam epitaxy can be monitored in real time during growth which should significantly enhance process control.

Chemisorption of HCl, Cl 2 and F 2 on the Si(100) surface

In an earlier publication we have presented the results of some preliminary calculations on the chemisorption of HF and F 2 onto the Si(100)2 × 1 surface using the SLAB-MINDO molecular orbital method. In this paper we extend this work to the case of HCl and Cl 2, and report on some new results for F 2. All of the species are assumed to dissociate and adsorb onto the surface silicon atoms, and the optimum configuration appropriate to a uniform monolayer coverage determined by minimising the total energy with respect to variations of the atomic coordinates within the six topmost surface layers. The behaviour of HCl on the Si(100) surface is found to be very similar to that obtained previously for HF. The H and Cl atoms chemisorb onto the dangling bonds of the surface silicon atoms at close to the expected tetrahedral angles whilst retaining a 2 × 1 dimer topology for the silicon substrate as suggested by experiment. The adsorption of Cl 2 is also found to yield a 2 × 1 symmetric dimer topology with each Si-Cl bond being tilted away from the surface normal at an angle of 14.8°. The lowest energy configuration for Cl 2 on the Si(100) surface, however, is determined to be a 2 × 1 bridge site model in which the two chlorine atoms are sited 0.708 Å and 1.132 Å above the surface plane. An analogous bridge site model is also found for F 2 chemisorption with the fluorine atoms lying 0.390 Å and 0.015 Å above the silicon surface. While this topology is in complete contrast to the molecular type topology reported earlier for a monolayer coverage of F 2 on the Si(100)2 × 1 surface, both configurations are found to have equivalent energies and might thus be expected to coexist during the early stages of fluorine chemisorption.