Ab initio density functional investigation of B_24 cluster: Rings, Tubes, Planes, and Cages

We investigate the equilibrium geometries and the systematics of bonding in various isomers of a 24-atom boron cluster using Born-Oppenheimer molecular dynamics within the framework of density functional theory. The isomers studied are the rings, the convex and the quasiplanar structures, the tubes and, the closed structures. A staggered double-ring is found to be the most stable structure amongst the isomers studied. Our calculations reveal that a 24-atom boron cluster does form closed 3-d structures. All isomers show staggered arrangement of nearest neighbor atoms. Such a staggering facilitates $sp^2$ hybridization in boron cluster. A polarization of bonds between the peripheral atoms in the ring and the planar isomers is also seen. Finally, we discuss the fusion of two boron icosahedra. We find that the fusion occurs when the distance between the two icosahedra is less than a critical distance of about 6.5a.u.Comment: 8 pages, 9 figures in jpeg format Editorially approved for publication in Phys. Rev.

تغيرات نظم الرياح وتأثيراتها المتوقعة على حركة البقع النفطية في منطقة الخليج العربي

The marine data required for the models of the oil spill movement prediction (current and wind data) are discussed. The available water current data in the Arabian Gulf can not be used with confidence because of lack of the direct current measurements. The hourly wind velocity data at Doha Airport in the period (1984-1988), are analysed to determine the daily and monthly mean vectors. These results are compared with wind roses at the same station in different years. Assuming that wind pattern is similar in Qatari waters, the wind is expected to generate currents, carrying the oil spill, towards the S-SE direction, except in August and September where the wind driven current could flow towards W to SW. Suggestions are proposed to obtain accurate input wind data to get more reliable forecasting of oil spill movement.في هذا البحث اشير إلى بعض النماذج الرياضية المستخدمة في التنبؤ بحركة البقعة النفطية في مناطق مختلفة بما فيها منطقة الخليج العربي كما نوقشت البيانات الأساسية المطلوبة لتطبيق هذه النماذج والتي تتعلق بنظم التيارات البحرية والرياح . وقد تبين من ذلك أن معظم بيانات التيارات البحرية المتواجدة بمنطقة الخليج العربي لم يتم معايرتها بقياسات مباشرة وبالتالي فإنه لا يمكن تقييم مدى دقتها . اما بالنسبة لنظم الرياح بالمنطقة فقد تم تحليل البيانات الساعية لسرعة الرياح عند مطار الدوحة وذلك في الفترة من يناير 1984 حتى ديسمبر 1988 باستخدام طريقة المتجهات وقد دلت النتائج على أنه مع افتراض أن نظام الرياح في مجال المياه القطرية يشبه النظام عند مطار الدوحة ، فإن البقعة النفطية تزاح تحت تأثير الرياح كما يلي : خلال معظم شهور العام يتوقع حركة البقعة النفطية في أتجاه الجنوب والجنوب الشرقي ( أي في اتجاه مواز تقريبا للسواحل القطرية ) ما عدا في شهري اغسطس وسبتمبر حيث يتجه التيار المتولد الناشئ عن الرياح في احيان كثيرة نحو الغرب والجنوب الغربي تجاه السواحل القطرية الشرقية. وقد تم اقتراح بعض الوسائل للحصول على قاعدة للبيانات البحرية اللازمة للحصول على تنبؤات أدق لحركة البقعة النفطية

Stretch reflexes of triceps surae in patients with upper motor neuron syndromes

Electromyographic responses of triceps surae to dorsiflexion stretch were studied in 47 patients with a variety of lesions producing an upper motor neuron syndrome. The short latency spinal reflexes, both when patient was at rest and when he was exerting a voluntary plantarflexion, were frequently enhanced in magnitude and the rate of increase with acceleration was also enhanced. Long-latency reflexes were uncommon at rest. With background force long-latency reflexes were present unless the short latency reflex was very large. Long latency reflexes often were normal, but in some patients they were either excessively large or even of abnormal shape with prolonged continuous activity. The clinical assessment of the ankle jerk correlated with the magnitude of the short latency reflex. The clinical assessment of tone correlated with the magnitude of the short latency reflex, the magnitude of the long latency reflex and the duration of the long latency reflex. There appear to be multiple physiological mechanisms underlying the clinical phenomenon of spasticity